Semiconductor device and healthcare system

ABSTRACT

Provided is a semiconductor device capable of reducing its area, operating at a high speed, or reducing its power consumption. A circuit  50  is used as a memory circuit with a function of performing an arithmetic operation. One of a circuit  80  and a circuit  90  has a region overlapping with at least part of the other of the circuit  80  and the circuit  90 . Accordingly, the circuit  50  can perform the arithmetic operation that is essentially performed in the circuit  60 ; thus, a burden of the arithmetic operation on the circuit  60  can be reduced. Moreover, the number of times of data transmission and reception between the circuits  50  and  60  can be reduced. Furthermore, the circuit  50  functioning as a memory circuit can have a function of performing an arithmetic operation while the increase in the area of the circuit  50  is suppressed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice having functions of storing data and performing an arithmeticoperation, and also relates to a healthcare system using thesemiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Another embodiment of the present inventionrelates to a semiconductor device, a display device, a lighting device,a power storage device, a memory device, or a driving method ormanufacturing method thereof.

2. Description of the Related Art

To monitor biological data of persons and animals, healthcare systemswhich determine body temperature, a pulse rate, and the like using asensor have been widely used.

A semiconductor device is usually used for the healthcare system, andthe semiconductor device includes a memory for storing biological data,a processor provided with a logic circuit for processing data stored inthe memory, and the like.

Patent Document 1 discloses an integrated circuit including a memoryarray and a logic circuit connected to the memory array.

PATENT DOCUMENT

[Patent Document 1] Japanese Published Patent Application No.2011-155264

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel semiconductor device. Another object of one embodiment of thepresent invention is to provide a semiconductor device with reducedarea. Another object of one embodiment of the present invention is toprovide a semiconductor device capable of operating at a high speed.Another object of one embodiment of the present invention is to providea semiconductor device capable of reducing power consumption.

One embodiment of the present invention does not necessarily achieve allthe objects listed above and only needs to achieve at least one of theobjects. The description of the above objects does not disturb theexistence of other objects. Other objects will be apparent from and canbe derived from the description of the specification, the drawings, theclaims, and the like.

A semiconductor device according to one embodiment of the presentinvention includes a first circuit, a second circuit, and a thirdcircuit. The first circuit has a function of acquiring data from theoutside. The second circuit has a function of converting the dataacquired in the first circuit into a digital signal. The third circuitincludes a fourth circuit including a memory circuit and a fifth circuitincluding an arithmetic circuit. The fourth circuit is provided over thefifth circuit. One of the fourth circuit and the fifth circuit has aregion overlapping with at least a part of the other of the fourthcircuit and the fifth circuit. The memory circuit includes a transistorincluding an oxide semiconductor in a channel formation region.

A semiconductor device according to one embodiment of the presentinvention includes a first circuit, a second circuit, and a thirdcircuit. The first circuit has a function of acquiring data from theoutside. The second circuit has a function of converting the dataacquired in the first circuit into a digital signal. The third circuitcomprises a fourth circuit including a first memory circuit and a secondmemory circuit and a fifth circuit including an arithmetic circuit. Thefourth circuit is provided over the fifth circuit. One of the fourthcircuit and the fifth circuit has a region overlapping with at least apart of the other of the fourth circuit and the fifth circuit. The firstmemory circuit includes a first transistor including an oxidesemiconductor in a channel formation region. The second memory circuitincludes a second transistor including an oxide semiconductor in achannel formation region. The first memory circuit has a function ofstoring biological data acquired in the first circuit. The second memorycircuit has a function of storing a reference value that is comparedwith the biological data. The fifth circuit has a function of comparingthe biological data and the reference value.

In the semiconductor device according to one embodiment of the presentinvention, the first memory circuit may include a first capacitor. Oneof a source and a drain of the first transistor may be connected to thefirst capacitor. The second memory circuit may include a secondcapacitor and an inverter. One of a source and a drain of the secondtransistor may be connected to the second capacitor and an inputterminal of the inverter. An output terminal of the inverter may beconnected to the fifth circuit.

The semiconductor device according to one embodiment of the presentinvention may further include a third transistor. One of a source and adrain of the third transistor may be electrically connected to the firstmemory circuit. The other of the source and the drain of the thirdtransistor may be electrically connected to the fifth circuit. The thirdtransistor may include an oxide semiconductor in a channel formationregion.

A healthcare system according to one embodiment of the present inventionincludes any one of the above semiconductor devices and has a functionof transmitting and receiving a wireless signal.

According to one embodiment of the present invention, a novelsemiconductor device can be provided. Alternatively, according to oneembodiment of the present invention, a semiconductor device with reducedarea can be provided. Alternatively, according to one embodiment of thepresent invention, a semiconductor device capable of operating at a highspeed can be provided. Further alternatively, a semiconductor devicecapable of reducing power consumption can be provided.

Note that the description of these effects does not disturb theexistence of other effects. In one embodiment of the present invention,there is no need to achieve all the effects. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C illustrate an example of a structure of a semiconductordevice.

FIGS. 2A and 2B illustrate examples of a structure of a semiconductordevice.

FIGS. 3A to 3D illustrate examples of a structure of a semiconductordevice.

FIG. 4 is a flowchart showing an operation of a semiconductor device.

FIG. 5 illustrates an example of a structure of a semiconductor device.

FIGS. 6A to 6C are circuit diagrams each illustrating an example of astructure of a semiconductor device.

FIGS. 7A to 7C are circuit diagrams each illustrating an example of astructure of a semiconductor device.

FIG. 8 is a circuit diagram illustrating an example of a structure of asemiconductor device.

FIG. 9 is a circuit diagram illustrating an example of a structure of asemiconductor device.

FIG. 10 is a circuit diagram illustrating an example of a structure of asemiconductor device.

FIGS. 11A and 11B are circuit diagrams each illustrating an example of astructure of a semiconductor device.

FIGS. 12A to 12D are circuit diagrams each illustrating an example of astructure of a semiconductor device.

FIGS. 13A to 13D illustrate an example of a structure of a transistor.

FIGS. 14A to 14C each illustrate an example of a structure of atransistor.

FIG. 15 illustrates an example of a structure of a transistor.

FIGS. 16A to 16C illustrate an example of a structure of a transistor.

FIGS. 17A to 17C illustrate an example of a structure of a transistor.

FIG. 18 illustrates an example of a structure of a transistor.

FIGS. 19A to 19E illustrate application examples of a semiconductordevice.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be describedbelow in detail with reference to the accompanying drawings. Note thatthe present invention is not limited to the following description and itis easily understood by those skilled in the art that the mode anddetails can be variously changed without departing from the scope andspirit of the present invention. Therefore, the present invention shouldnot be construed as being limited to the description in the followingembodiments.

One embodiment of the present invention includes, in its category,devices including an integrated circuit, such as a radio frequency (RF)tag and a display device. The display device includes, in its category,a display device including an integrated circuit, such as a liquidcrystal display device, a light-emitting device in which alight-emitting element typified by an organic light-emitting element isprovided in each pixel, an electronic paper, a digital micromirrordevice (DMD), a plasma display panel (PDP), and a field emission display(FED).

In describing structures of the present invention with reference to thedrawings, common reference numerals are used for the same portions indifferent drawings.

Note that in this specification and the like, part of a diagram or atext described in one embodiment can be taken out to constitute oneembodiment of the invention. Thus, in the case where a diagram or a textrelated to a certain portion is described, the context taken out frompart of the diagram or the text is also disclosed as one embodiment ofthe invention, and one embodiment of the invention can be constituted.The embodiment of the present invention is clear. Therefore, forexample, in a diagram or a text in which one or more active elements(e.g., transistors), wirings, passive elements (e.g., capacitors),conductive layers, insulating layers, semiconductor layers, components,devices, operating methods, manufacturing methods, or the like aredescribed, part of the diagram or the text is taken out, whereby oneembodiment of the invention can be constituted. For example, from acircuit diagram in which N circuit elements (e.g., transistors orcapacitors; N is an integer) are provided, it is possible to constituteone embodiment of the invention by taking out M circuit elements (e.g.,transistors or capacitors; M is an integer, where M<N). For anotherexample, it is possible to take out some given elements from a sentence“A includes B, C, D, E, or F” and constitute one embodiment of theinvention, for example, “A includes B and E”, “A includes E and F”, “Aincludes C, E, and F”, or “A includes B, C, D, and E”.

Note that in the case where at least one specific example is describedin a diagram or text described in one embodiment in this specificationand the like, it will be readily appreciated by those skilled in the artthat a broader concept of the specific example can be derived.Therefore, in the diagram or the text described in one embodiment, inthe case where at least one specific example is described, a broaderconcept of the specific example is disclosed as one embodiment of theinvention, and one embodiment of the invention can be constituted. Theembodiment of the present invention is clear.

Note that, in this specification and the like, a content described in atleast a diagram (which may be part of the diagram) is disclosed as oneembodiment of the invention and can constitute one embodiment of theinvention. Therefore, when a certain content is described in a diagram,the content is disclosed as one embodiment of the invention even whenthe content is not described with a text, and one embodiment of theinvention can be constituted. In a similar manner, part of a diagram,which is taken out from the diagram, is disclosed as one embodiment ofthe invention, and one embodiment of the invention can be constituted.The embodiment of the present invention is clear.

When the range of a value that is defined by the maximum and minimumvalues is described, the range may be appropriately narrowed or part ofthe range may be excluded, whereby one embodiment of the inventionexcluding part of the range can be constructed. In this manner, it ispossible to specify the technical scope of one embodiment of the presentinvention so that a conventional technology is excluded, for example.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor), a passive element (e.g., a capacitor), or the likeare connected are not specified. In other words, one embodiment of theinvention can be clear even when connection portions are not specified.Further, in the case where a connection portion is disclosed in thisspecification and the like, it can be determined that one embodiment ofthe invention in which a connection portion is not specified isdisclosed in this specification and the like, in some cases. Inparticular, in the case where the number of portions to which theterminal is connected might be plural, it is not necessary to specifythe portions to which the terminal is connected. Therefore, it might bepossible to constitute one embodiment of the invention by specifyingonly portions to which some of terminals of an active element (e.g., atransistor), a passive element (e.g., a capacitor), or the like areconnected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the presentinvention can be clear. Further, it can be determined that oneembodiment of the present invention whose function is specified isdisclosed in this specification and the like. Therefore, when aconnection portion of a circuit is specified, the circuit is disclosedas one embodiment of the invention even when a function is notspecified, and one embodiment of the invention can be constituted.Alternatively, when a function of a circuit is specified, the circuit isdisclosed as one embodiment of the invention even when a connectionportion is not specified, and one embodiment of the invention can beconstituted.

Embodiment 1

In this embodiment, a structure example of one embodiment of the presentinvention is described.

FIG. 1A illustrates a structure example of a semiconductor device 10 ofone embodiment of the present invention. The semiconductor device 10includes circuits 20, 30, 40, 50, 60, and 70.

In one embodiment of the present invention, the circuit 50 can be usedas a memory circuit with a function of performing an arithmeticoperation. The circuit 50 can output to the circuit 60 data stored inthe circuit 50, data input from the circuit 40, and data obtained as aresult of the arithmetic operation using these data as an input signal.The circuit 50 can perform the arithmetic operation that is essentiallyperformed in the circuit 60; thus, a burden of the arithmetic operationon the circuit 60 can be reduced. Furthermore, the number of times ofdata transmission and reception between the circuits 50 and 60 can bereduced. Thus, the operation speed of the semiconductor device 10 can beimproved. Each circuit in FIG. 1A is described below.

The circuit 20 has a function of acquiring data from the outside. Thecircuit 20 includes a sensor which has a function of acquiringpredetermined physical quantities or predetermined chemical quantities,and the like.

Further, the physical quantities mentioned here indicate temperature,pressure, a flow rate, light, magnetism, a sound wave, acceleration,humidity, and the like. The chemical quantities mentioned here indicatequantities of chemical substances such as a gas component like a gas anda liquid component like an ion, and the like. In addition to the above,the chemical quantities further include quantities of organic compoundslike a specific biological material contained in blood, sweat, urine,and the like. In particular, in order to acquire a chemical quantity, aspecific substance is acquired selectively, and therefore, a substancewhich reacts with the specific substance to be acquired is provided inthe circuit 20 in advance. For example, in the case of acquiring abiological material, it is preferable that an enzyme, a resistormolecule, a microbial cell, or the like which reacts with the biologicalmaterial that is to be acquired be fixed in a polymer molecule or thelike and provided in the circuit 20.

The circuit 20 preferably has a function of acquiring biological data ofpersons or animals. Examples of the biological data include bodytemperature, blood pressure, a pulse rate, the amount of sweat, lungcapacity, a blood sugar level, the number of white blood cells, thenumber of red blood cells, the number of platelets, hemoglobinconcentration, a hematocrit value, a GOT(AST) value, a GPT(ALT) value, aγ-GTP value, an LDL cholesterol value, an HDL cholesterol value, aneutral fat value, and the like. The circuit 20 has a function ofacquiring biological data; thus, the semiconductor device 10 can be usedas a healthcare system.

The circuit 30 has a function of controlling acquisition of data in thecircuit 20. The circuit 30 can be formed using a timer which has afunction of controlling the frequency and timing at which the circuit 20acquires data from the outside, and the like. Further, the circuit 30can measure time at which the circuit 20 has acquired data from theoutside, and can output the time to the circuit 50.

The circuit 40 has a function of converting data acquired in the circuit20 into a digital signal. The circuit 40 can be formed using an ADconverter which has a function of converting an analog signalcorresponding to biological data input from the circuit 20 into adigital signal.

The circuit 50 has a function of storing data input from the circuit 40.Further, the circuit 50 has a function of performing an arithmeticoperation using the data input from the circuit 40 or data stored in thecircuit 50 as an input signal. That is, the circuit 50 can be used as amemory circuit with a function of performing an arithmetic operation.

Specifically, the circuit 50 includes a circuit 80 and a circuit 90. Thecircuit 80 includes a circuit which has a function of storing data(hereinafter also referred to as a memory circuit). The circuit 80 canbe formed using a cell array including a plurality of memory circuits.The memory circuit can be formed using a volatile memory cell such as aDRAM cell or an SRAM cell, or a nonvolatile memory cell such as an EPROMcell or an MRAM cell. In particular, the memory circuit is preferablyformed using a transistor including an oxide semiconductor in itschannel formation region (hereinafter also referred to as an OStransistor).

An oxide semiconductor has a wider band gap and lower intrinsic carrierdensity than silicon and the like. Thus, the off-state current of the OStransistor is extremely low. As a result, the use of the OS transistorfor the memory circuit included in the circuit 80 enables the datastored in the memory circuit to be held for a long time.

The OS transistor can operate at a high speed when miniaturized. Thus,the operation speed of the memory circuit included in the circuit 80 canbe increased with the use of the OS transistor for the memory circuitincluded in the circuit 80.

The circuit 90 includes a circuit which has a function of performing anarithmetic operation (hereinafter also referred to as an arithmeticcircuit). The arithmetic circuit can be formed using a logic circuitsuch as a NOT circuit, an AND circuit, an OR circuit, a NAND circuit, aNOR circuit, an XOR circuit, or an XNOR circuit. Further, a comparisoncircuit, an adder circuit, a subtractor circuit, a multiplier circuit, adivider circuit, or the like may be formed by combining any of theselogic circuits.

The circuit 90 has a function of performing an arithmetic operationusing data input from the circuit 40 or data stored in the circuit 80 asan input signal. For example, in the case where the circuit 90 includesa comparison circuit, the data input from the circuit 40 and the datastored in the circuit 80 can be compared with each other. In the casewhere the data input from the circuit 40 is biological data acquired inthe circuit 20 and the data stored in the circuit 80 is a predeterminedreference value, it can be determined whether the biological data isnormal or abnormal by comparing the biological data and the referencevalue in the circuit 90. Note that a structure may be employed in whichdata input from the circuit 40 is once stored in the circuit 80 and anarithmetic operation using the data as an input signal is performed.

In the case where the circuit 90 includes a subtractor circuit, adifference between data input from the circuit 40 and data stored in thecircuit 80 can be calculated. Further, in the case where the circuit 90includes an adder circuit and a divider circuit, the average value ofdata input from the circuit 40 and data stored in the circuit 80 can becalculated. In the case where the data input from the circuit 40 isbiological data acquired in the circuit 20 and the data stored in thecircuit 80 is biological data acquired before, variation and the averagevalue of the biological data can be calculated in the circuit 90.

As shown in FIG. 1B, the circuit 80 has a function of storing data inputfrom the circuit 40 or data obtained by an arithmetic operation in thecircuit 90. The circuit 80 has a function of outputting the data storedin the circuit 80 to the circuit 90 or the circuit 60. The circuit 90has a function of performing an arithmetic operation using the datainput from the circuit 40 or the data stored in the circuit 80 as aninput signal. Further, the circuit 90 has a function of outputting aresult of the arithmetic operation to the circuit 80 or the circuit 60.

The circuit 60 has functions of performing data processing, controllinganother circuit, and the like. The circuit 60 can be formed using aprocessor and the like including a variety of logic circuits such as asequential circuit and a combination circuit which include a pluralityof transistors. Note that in one embodiment of the present invention,the circuit 50 includes the circuit 90 including an arithmetic circuit.Thus, the arithmetic operation that is essentially performed in thecircuit 60 (specifically, the arithmetic operation using the data storedin the circuit 80 as an input signal) can be performed in the circuit50. Therefore, access to the data stored in the circuit 50 from thecircuit 60 and writing of the result of the arithmetic operation by thecircuit 60 to the circuit 50 can be omitted, resulting in a reduction inthe number of times of data transmission and reception between thecircuit 50 and the circuit 60.

The circuit 70 is a communication circuit having a function oftransmitting and receiving a signal. The circuit 70 is controlled by thecircuit 60, and is capable of transmitting the data stored in thecircuit 80 or the result of the arithmetic operation by the circuit 90to the outside of the semiconductor device 10. The data transmitted fromthe circuit 70 can be read out with a computer provided outside thesemiconductor device or a reader/writer, or the like.

Note that the transmission and reception of a signal in the circuit 70may be performed via wire or wirelessly. In the case where thetransmission and reception of a signal in the circuit 70 are performedusing a wireless signal, the semiconductor device 10 can be used as awearable healthcare system that can be worn on clothing or a human body.

As described above, in one embodiment of the present invention, thecircuit 50 can be used as a memory circuit with a function of performingan arithmetic operation. The circuit 50 can output to the circuit 60data stored in the circuit 50, data input from the circuit 40, and dataobtained as a result of the arithmetic operation using these data as aninput signal. The circuit 50 can perform the arithmetic operation thatis essentially performed in the circuit 60; thus, a burden of thearithmetic operation on the circuit 60 can be reduced. Furthermore, thenumber of times of data transmission and reception between the circuits50 and 60 can be reduced. Thus, the operation speed of the semiconductordevice 10 can be increased.

FIG. 1C is a schematic view of a cross sectional structure of thecircuit 50. The circuit 50 includes the circuit 90 over a substrate 100,an insulating layer 101 over the circuit 90, and the circuit 80 over theinsulating layer 101. That is, the circuit 50 has a structure in whichthe circuit 90 and the circuit 80 are stacked. The insulating layer 101has an opening, and a conductive layer 102 is provided in the opening.The circuit 90 is connected to the circuit 80 through the conductivelayer 102.

Here, one of the circuit 80 and the circuit 90 preferably has a regionoverlapping with at least part of the other of the circuit 80 and thecircuit 90. In that case, the circuit 50 which functions as a memorycircuit can have an additional function of performing an arithmeticoperation while the increase in the area of the circuit 50 issuppressed. Thus, the area of the semiconductor device 10 can bereduced. When one of the circuit 80 and the circuit 90 has a regionoverlapping with an entire surface of the other of the circuit 80 andthe circuit 90, the area of the circuit 50 can be further reduced.

Note that in this specification and the like, when it is explicitlydescribed that X and Y are connected, the case where X and Y areelectrically connected, the case where X and Y are functionallyconnected, and the case where X and Y are directly connected areincluded therein. Accordingly, another element may be interposed betweenelements having a connection relation shown in drawings and texts,without limiting to a predetermined connection relation, for example,the connection relation shown in the drawings and the texts. Here, X andY each denote an object (e.g., a device, an element, a circuit, a line,an electrode, a terminal, a conductive film, a layer, or the like).

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. A switch is controlled to be on or off. Thatis, a switch is conducting or not conducting (is turned on or off) todetermine whether current flows therethrough or not. Alternatively, theswitch has a function of selecting and changing a current path.

For example, example, in the case where X and Y are functionallyconnected, one or more circuits that enable functional connectionbetween X and Y (e.g., a logic circuit such as an inverter, a NANDcircuit, or a NOR circuit; a signal converter circuit such as a D/Aconverter circuit, an A/D converter circuit, or a gamma correctioncircuit; a potential level converter circuit such as a power supplycircuit (e.g., a step-up circuit or a step-down circuit) or a levelshifter circuit for changing the potential level of a signal; a voltagesource; a current source; a switching circuit; an amplifier circuit suchas a circuit that can increase signal amplitude, the amount of current,or the like, an operational amplifier, a differential amplifier circuit,a source follower circuit, and a buffer circuit; a signal generationcircuit; a memory circuit; or a control circuit) can be connectedbetween X and Y. Note that, for example, in the case where a signaloutput from X is transmitted to Y even when another circuit isinterposed between X and Y, X and Y are functionally connected.

Note that when it is explicitly described that X and Y are electricallyconnected, the case where X and Y are electrically connected (i.e., thecase where X and Y are connected with another element or another circuitprovided therebetween), the case where X and Y are functionallyconnected (i.e., the case where X and Y are functionally connected withanother circuit provided therebetween), and the case where X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween) are includedtherein. That is, when it is explicitly described that “X and Y areelectrically connected”, the description is the same as the case whereit is explicitly only described that “X and Y are connected”.

Note that, for example, the case where a source (or a first terminal orthe like) of a transistor is electrically connected to X through (or notthrough) Z1 and a drain (or a second terminal or the like) of thetransistor is electrically connected to Y through (or not through) Z2,or the case where a source (or a first terminal or the like) of atransistor is directly connected to one part of Z1 and another part ofZ1 is directly connected to X while a drain (or a second terminal or thelike) of the transistor is directly connected to one part of Z2 andanother part of Z2 is directly connected to Y, can be expressed by usingany of the following expressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit configuration is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope. When the connection order in a circuit configuration isdefined by an expression similar to the above examples, a source (or afirst terminal or the like) and a drain (or a second terminal or thelike) of a transistor can be distinguished from each other to specifythe technical scope. Note that these expressions are examples and thereis no limitation on the expressions. Note that these expressions areexamples and there is no limitation on the expressions. Here, X, Y, Z1,and Z2 each denote an object (e.g., a device, an element, a circuit, awiring, an electrode, a terminal, a conductive film, and a layer).

Even when independent components are electrically connected to eachother in the drawing, one component has functions of a plurality ofcomponents in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

The circuit 90 can be formed using a transistor whose channel formationregion is formed in part of the substrate 100. In this case, thesubstrate 100 preferably includes a single crystal semiconductor. Asingle crystal silicon substrate, a single crystal germanium substrate,or the like can be used as the substrate 100. By using a substrateincluding a single crystal semiconductor as the substrate 100, thecircuit 90 can be formed using a transistor including a single crystalsemiconductor in its channel formation region. The transistor includinga single crystal semiconductor in its channel formation region has ahigher current supply capability; thus formation of the circuit 90 usingsuch a transistor improves the operation speed of the circuit 90.

Next, an example of the structure of the circuit 50 is described withreference to FIGS. 2A and 2B.

FIG. 2A is a perspective view illustrating an example of the circuit 50in FIGS. 1A to 1C. The circuit 50 includes the circuit 90 and circuits110 and 120 which are formed over the substrate 100, the insulatinglayer 101 formed over the circuits 90, 110, and 120, and the circuit 80formed over the insulating layer 101. The circuit 80 includes aplurality of memory circuits 81.

The circuit 90 includes an arithmetic circuit, and is connected to thememory circuits 81. The circuit 90 can perform an arithmetic operationusing data stored in the memory circuits 81 as an input signal andoutput a result of the arithmetic operation to the circuit 60 (see FIG.1B). Note that the circuit 90 can also perform an arithmetic operationusing data input from the outside of the circuit 50 (e.g., the circuit40 in FIG. 1B) as an input signal.

The circuit 110 is a driver circuit which has a function of selecting aspecific memory circuit 81 from the plurality of memory circuits 81.Specifically, the circuit 110 has a function of supplying a wiringconnected to the specific memory circuit 81 with a signal (hereinafteralso referred to as a selection signal) for selecting the specificmemory circuit 81.

The circuit 120 is a driver circuit which has functions of writing datato the memory circuits 81 and reading data stored in the memory circuits81. Specifically, the circuit 120 has a function of supplying apotential (hereinafter also referred to as a writing potential)corresponding to data to be written to the specific memory circuit 81 tothe wiring connected to the specific memory circuit 81. Further, thecircuit 120 has a function of reading data stored in the specific memorycircuit 81 in accordance with a potential of the wiring connected to thespecific memory circuit 81. Note that the circuit 120 may have aprecharge function for supplying a predetermined potential to the wiringconnected to the memory circuit 81.

Here, the substrate 100 is preferably a substrate including a singlecrystal semiconductor. Thus, the circuits 90, 110, and 120 can be formedusing transistors each including a single crystal semiconductor in itschannel formation region. Accordingly, the operation speed of thecircuits 90, 110, and 120 can be improved.

The circuit 80 can be formed using a cell array including the pluralityof memory circuits 81 as memory cells. Note that each of the pluralityof memory circuits 81 is connected to the circuits 90, 110, and 120.

Here, each memory circuit 81 can be formed using a transistor whosechannel formation region is formed in a semiconductor film. For example,the memory circuit 81 can be formed using a transistor including anon-single-crystal semiconductor in its channel formation region. As thenon-single-crystal semiconductor, non-single-crystal silicon such asamorphous silicon, microcrystalline silicon, or polycrystalline silicon,non-single-crystal germanium such as amorphous germanium,microcrystalline germanium, or polycrystalline germanium, or the likecan be used. Furthermore, the memory circuit 81 can be formed using anOS transistor. The transistor whose channel formation region is formedin the above-described semiconductor film can be formed over theinsulating layer 101; thus, the memory circuit 81 can be formed over theinsulating layer 101. Thus, the circuit 50 can have a structure in whichthe circuit 80 and the circuit 90 are stacked.

In particular, the memory circuit 81 is preferably formed using an OStransistor. The OS transistor has extremely low off-state current; thus,the use of the OS transistor for the memory circuit 81 allows datastored in the memory circuit 81 to be held for a long period even afterthe supply of power to the circuit 80 is stopped. Thus, the memorycircuit 81 can be used as a nonvolatile memory cell or a memory cellwith an extremely low refresh frequency.

Furthermore, the OS transistor can operate at a high speed whenminiaturized. Thus, the operation speed of the memory circuit 81 can beimproved by the use of the OS transistor for the memory circuit 81.Specifically, the writing speed and the reading speed of the memorycircuit 81 can be less than or equal to 10 ns, preferably less than orequal to 5 ns, further preferably less than or equal to 1 nm. Note thatthe channel length of the OS transistor can be less than or equal to 100nm, preferably less than or equal to 60 nm, further preferably less thanor equal to 40 nm, further more preferably less than or equal to 30 nm.

Here, the circuit 90 preferably has a region overlapping with thecircuit 80. Specifically, the circuit 90 preferably has a regionoverlapping with at least any of the plurality of memory circuits 81.Accordingly, the circuit 50 which functions as a memory circuit can havean additional function of performing an arithmetic operation while theincrease in the area of the circuit 50 is suppressed. Note that thecircuit 90 is positioned so as to have region overlapping with all ofthe plurality of memory circuits 81; thus, the area of the circuit 50can be further reduced. Further, the circuit 110 or the circuit 120 canbe positioned so as to have a region overlapping with at least any ofthe plurality of memory circuits 81.

Note that in FIG. 2A, one layer of the circuit 80 including theplurality of memory circuits 81 is provided; however, such a circuitwith two or more layers may be provided. For example, a structure may beemployed in which an insulating layer is provided over the circuit 80and a circuit including the plurality of memory circuits 81 is providedthereover. With this structure, high capacity of the memory circuit canbe achieved while the increase in the area of the circuit 50 issuppressed.

FIG. 2A illustrates an example in which the circuit 110 and the circuit120 are provided over the substrate 100; however, the structure is notlimited to this example. The circuit 110 and the circuit 120 may beprovided over the insulating layer 101 (FIG. 2B). In this case, thecircuit 110 and the circuit 120 can be formed using transistors whosechannel formation regions are formed in a semiconductor film. Inparticular, the circuit 110 and the circuit 120 are preferably formedusing OS transistors which have small off-state current and are capableof high-speed operation.

FIGS. 3A to 3D illustrate examples of a top view of the circuit 50. FIG.3C corresponds to a top view of the structure (see FIG. 2A) in which thecircuit 110 and the circuit 120 are provided over the substrate 100.FIG. 3D corresponds to a top view of the structure (see FIG. 2B) inwhich the circuit 110 and the circuit 120 are provided over theinsulating layer 101.

As illustrated in FIG. 3A, the circuit 80 may be placed so as to have aregion overlapping with an entire surface of the circuit 90.Accordingly, the increase in the area of the circuit 50 can besuppressed as compared to the case where the circuit 80 and the circuit90 are formed on the same plane. Note that the circuit 80 may bepositioned so as to have a region overlapping with part of the circuit90.

As shown in FIG. 3B, the circuit 90 may be placed so as to have a regionoverlapping with an entire surface of the circuit 80. Note that thecircuit 90 may be positioned so as to have a region overlapping withpart of the circuit 80.

As shown in FIG. 3C, the circuit 80 may be placed so as to have regionsoverlapping with the entire surface of the circuit 90, the entiresurface of the circuit 110, and the entire surface of the circuit 120.In this case, while the increase in the area of the circuit 50 issuppressed, the area of the circuit 80 can be larger than that in thestructures illustrated in FIGS. 3A and 3B. With this structure, highcapacity of the memory circuit 80 which function as a memory circuit canbe achieved. Note that the circuit 80 may be placed so as to overlapwith part of the circuit 110, or may be placed so as to overlap withpart of the circuit 120.

As shown in FIG. 3D, the circuit 90 can also be placed so as to haveregions overlapping with the entire surfaces of the circuit 80, thecircuit 110, and the circuit 120. In this case, while the increase inthe area of the circuit 50 is suppressed, the area of the circuit 90 canbe larger than that in the structures illustrated in FIGS. 3A and 3B.Accordingly, the number and the kinds of arithmetic circuits included inthe circuit 90 can be increased. Thus, the operation speed of thecircuit 90 can be improved, and the kind of the arithmetic operationscan be increased. Note that the circuit 90 may be placed so as tooverlap with part of the circuit 110; alternatively, the circuit 90 maybe placed so as to overlap with part of the circuit 120.

Next, an example of operation of the semiconductor device 10 in FIG. 1is described with reference to a flowchart of FIG. 4. Here, descriptionis given of the case where the semiconductor device 10 is used as ahealthcare system which can determine whether the acquired biologicaldata is normal or abnormal.

First, the circuit 20 is controlled by the circuit 30 so that biologicaldata is acquired (Step S1). Then, an analog signal corresponding to theacquired biological data is converted into a digital signal in thecircuit 40 (Step S2).

Next, in the circuit 50, whether the biological data is normal orabnormal is determined (Step S3). This determination is performed in thecircuit 90 by comparison between a value of the biological data inputfrom the circuit 40 to the circuit 50 and a reference value previouslystored in the circuit 80. For example, in the case of measuring a bloodsugar level (BS), a predetermined blood sugar level (e.g., BS=126(mg/dl)) is stored in the circuit 80 as a reference value. Then, thevalue of a blood sugar level input from the circuit 40 is compared withthe reference value. When the blood sugar level is less than thereference value, it is determined as normal. When the blood sugar levelis greater than or equal to the reference value, it is determined asabnormal.

When the biological data is determined to be normal as a result of thedetermination in Step S3, data processing is performed in the circuit 90(Step S4). Examples of the data processing in the circuit 90 includecalculation of variation in the biological data and calculation of anaverage value of the biological data.

The variation in the biological data can be obtained by calculating adifference between a value of the biological data acquired by a certaintime and a value of the biological data acquired before the certaintime. The calculation of the difference can be performed by provision ofa subtractor circuit as the arithmetic circuit in the circuit 90.

Further, the average value of the biological data can be obtained bycalculating the sum of the values of the biological data acquired by thecertain time and dividing the value by the number of the acquiredbiological data. Note that the average value can be calculated byprovision of an adder circuit or a divider circuit as the arithmeticcircuit in the circuit 90.

After that, the result obtained through the data processing is stored inthe circuit 80 (Step S5). Note that the data stored in the circuit 80can be transmitted from the circuit 70 to the outside by the control ofthe circuit 60.

Note that the data processing in Step S4 can be omitted in the casewhere the biological data input from the circuit 40 is directly storedin the circuit 80 or output to the circuit 60 without performing dataprocessing in the circuit 90.

When the biological data is determined to be abnormal as a result of thedetermination in Step S3, a signal indicating abnormality (hereinafteralso referred to as an interrupt signal) is output to the circuit 60from the circuit 50 (Step S6). Then, the circuit 60 that has receivedthe interrupt signal controls the circuit 70 and the circuit 70transmits a signal indicating that the abnormality is acquired to theoutside (Step S7).

Also in the case where the biological data is abnormal, data processing(Step S8) that is similar to that in the case where the biological datais normal and data storing (Step S9) in the circuit 80 can be performed.At this time, a value of the biological data that is determined to be anabnormal value, time at which the abnormal value is acquired, and thelike can be stored in the circuit 80. The above-described data can betransmitted to the outside from the circuit 70.

As described above, in one embodiment of the present invention, thecircuit 50 can be used as a memory circuit with a function of performingan arithmetic operation. The circuit 50 can output to the circuit 60data stored in the circuit 50, data input from the circuit 40, and dataobtained as a result of the arithmetic operation using these data as aninput signal. The circuit 50 can perform the arithmetic operation thatis essentially performed in the circuit 60, thus, a burden of thearithmetic operation on the circuit 60 can be reduced. Further, thenumber of times of data transmission and reception between the circuits50 and 60 can be reduced. Thus, the operation speed of the semiconductordevice 10 can be improved.

Further, in one embodiment of the present invention, a structure can beemployed in which one of the circuit 80 and the circuit 90 has a regionoverlapping with at least part of the other of the circuit 80 and thecircuit 90. Thus, the circuit 50 which functions as a memory circuit canhave an additional function of performing an arithmetic operation whilethe increase in the area of the circuit 50 is suppressed. Thus, the areaof the semiconductor device 10 can be reduced.

This embodiment can be combined with any of the other embodiments asappropriate. Note that content (or may be part of the content) describedin one embodiment may be applied to, combined with, or replaced bydifferent content (or may be part of the different content) described inthe embodiment and/or content (or may be part of the content) describedin one or more different embodiments. Note that in each embodiment, acontent described in the embodiment is a content described withreference to a variety of diagrams or a content described with a textdescribed in this specification. In addition, by combining a diagram (orpart thereof) described in one embodiment with another part of thediagram, a different diagram (or part thereof) described in the sameembodiment, and/or a diagram (or part thereof) described in one or aplurality of different embodiments, much more diagrams can be formed.This applies also to other embodiments.

Embodiment 2

In this embodiment, a specific structure example of one embodiment ofthe present invention is described. Here, a structure of thesemiconductor device 10 having a function as a healthcare system whichdetermines whether the acquired biological data is normal or abnormal isdescribed.

FIG. 5 illustrates an example of the structure of the circuit 50. Thecircuit 50 includes the circuits 80, 90, 110, 120, and a plurality ofcircuits 83. Note that the circuits 80 and 90 are formed on the samesurface in FIG. 5 for the convenience of explanation; however, thecircuits 80 and 90 are formed so as to overlap with each other as shownin FIGS. 1A to 1C, FIGS. 2A and 2B, and FIGS. 3A to 3D.

The circuit 80 includes a plurality of memory circuits 81 and aplurality of memory circuits 82. Here, a structure is shown in which thecircuit 80 includes n rows and m columns (m and n are natural numbers)of memory circuits 81 (memory circuits 81 [1, 1] to [n, m]) and one rowand m columns of memory circuits 82 (memory circuits 82 [1] to [m]).Note that two or more columns of memory circuits 82 may be provided. Thememory circuits 81 and the memory circuits 82 function as memory cells,and the circuit 80 functions as a cell array including a plurality ofmemory cells.

Here, the circuit 80 includes n rows and m columns of memory circuits81; thus, n kinds of m-bit data can be stored. Thus, n kinds of valuesof m-bit biological data acquired at different times and under differentconditions can be stored. Note that the number of the columns (m) of thememory circuit 81 can be freely determined depending on biological datato be acquired. For example, in the case of acquiring a blood sugarlevel as biological data, numerical values in the range where the bloodsugar level (BS) is 0 to 255 (mg/dl) can be stored when m is eight.

The memory circuits 81 are preferably formed using OS transistors. Inthat case, biological data stored in the memory circuits 81 can be heldfor a long period, and the memory circuits 81 can be used as nonvolatilememory cells or memory cells with an extremely low refresh frequency.

Furthermore, one kind of m-bit data can be stored in one row and mcolumns of memory circuits 82 included in the circuit 80. Here, in thememory circuits 82 [1] to [m], m-bit data that is a reference value ofbiological data can be stored. This reference value can be, for example,the value of a border between a normal value and an abnormal value (theupper limit or the lower limit of the normal value) of the biologicaldata. For example, when the blood sugar level (BS) is acquired asbiological data, the upper limit where the blood sugar level (BS) is 126mg/dl of the normal value can be stored.

An example is described in which the memory circuits 82 are provided inone row; however, the memory circuits 82 may be provided in a pluralityof rows. In that case, because a plurality of reference values can bestored, the upper limit and the lower limit of the biological data, or aplurality of upper limits or a plurality of lower limits can be storedin the memory circuits 82. Note that the number of rows of the memorycircuits 82 is not specifically limited, and an arbitrary number whichis one or more can be selected.

For example, when a blood sugar level is acquired as biological data,the memory circuits 82 are provided in three rows and m columns, and afirst upper limit (e.g., BS=110 (mg/dl)), a second upper limit (e.g.,BS=116 (mg/dl)), and a third upper limit (e.g., BS=126 (mg/dl)) can bestored in the first row, the second row, and the third row of the memorycircuits 82, respectively. Thus, the acquired blood sugar level can becompared with the first to the third upper limits, and abnormality ofthe biological data can be determined in stages.

In particular, the memory circuit 82 is preferably formed using an OStransistor. The OS transistor has extremely low off-state current; thus,the use of the OS transistor for the memory circuit 82 allows datastored in the memory circuit 82 to be held for a long period even afterthe supply of power to the circuit 80 is stopped. Thus, the memorycircuit 82 can be used as a nonvolatile memory cell or a memory cellwith an extremely low refresh frequency. Thus, after writing of thereference value to the memory circuit 82 is performed once, thereference value can be held for a long period even in a period duringwhich the supply of power to the circuit 80 is stopped.

The circuit 110 is connected to the memory circuits 81 through aplurality of wirings 111 (wirings 111 [1] to [n]). Further, the circuit110 is connected to the memory circuits 82 through a wiring 112. Thecircuit 110 is a driver circuit which has a function of supplying aselection signal to the wirings 111 or the wiring 112.

The circuit 120 is connected to the memory circuits 81 and the memorycircuits 82 through a plurality of wirings 121 (wirings 121 [1] to [m]).The circuit 120 is a driver circuit which has a function of supplying tothe wiring 121 a potential corresponding to data to be written to thememory circuits 81 or the memory circuits 82 and a function of readingout data stored in the memory circuits 81 or the memory circuits 82 inaccordance with the potential of the wiring 121. Note that the circuit120 may have a precharge function of supplying a predetermined potentialto the wiring 121.

Each of the plurality of circuits 83 (the circuits [1] to [m]) isconnected to the wiring 113, the wiring 121, and the circuit 90. Eachcircuit 83 has a function as a switch which controls output of datastored in the memory circuits 81 to the circuit 90. The conduction stateof the circuit 83 is controlled with a potential of the wiring 113, andthe circuit 83 is brought into a conduction state when an arithmeticoperation is performed in the circuit 90.

The circuit 83 can be formed using a transistor, for example. When thecircuit 83 is formed using a transistor, a gate of the transistor isconnected to the wiring 113, one of a source and a drain thereof isconnected to the wiring 121, and the other of the source and the drainthereof is connected to the circuit 90. In that case, the on/off stateof the transistor is controlled with a potential of the wiring 113. Whenthe transistor is turned on, the data stored in the memory circuits 81is output to the circuit 90, and an arithmetic operation can beperformed in the circuit 90.

When the circuit 83 is formed using a transistor, an OS transistor canbe used. The OS transistors have extremely low off-state current; thus,charge transfer between the wiring 121 and the circuit 90 can bedrastically suppressed in a period where the arithmetic operation is notperformed in the circuit 90, that is, in a period when the OS transistoris turned off.

Note that a “source” of a transistor in this specification means asource region that is part of a semiconductor film functioning as anactive layer or a source electrode connected to the semiconductor film.Similarly, a “drain” of the transistor means a drain region that is partof the semiconductor film or a drain electrode connected to thesemiconductor film. A “gate” means a gate electrode.

The terms “source” and “drain” of a transistor interchange with eachother depending on the conductivity type of the transistor or levels ofpotentials applied to the terminals. In general, in an n-channeltransistor, a terminal to which a lower potential is applied is called asource, and a terminal to which a higher potential is applied is calleda drain. Further, in a p-channel transistor, a terminal to which a lowerpotential is applied is called a drain, and a terminal to which a higherpotential is applied is called a source. In this specification, althoughconnection relation of the transistor is described assuming that thesource and the drain are fixed in some cases for convenience, actually,the names of the source and the drain interchange with each otherdepending on the relation of the potentials.

The circuit 90 includes circuits 91 and 92. The circuit 91 is a circuitwhich has a function of performing an arithmetic operation, and includesone or more arithmetic circuits. The arithmetic circuit can be formedusing a logic circuit such as a NOT circuit, an AND circuit, an ORcircuit, a NAND circuit, a NOR circuit, an XOR circuit, or an XNORcircuit. Further, a comparison circuit, an adder circuit, a subtractorcircuit, a multiplier circuit, a divider circuit, or the like may beformed by combining any of these logic circuits. Here, the case wherethe circuit 91 includes the comparison circuit is described.

The circuit 91 is connected to the memory circuits 82, the circuits 83,and the circuit 92. To the circuit 91, data stored in the memory circuit81 is input through the circuits 83, and data stored in the memorycircuit 82 is input. The circuit 91 has a function of comparing thesizes of these data, and outputting a signal corresponding to thecomparison result to the circuit 92. For example, the circuit 91 cancompare data stored in any of the memory circuits 81 [1, 1] to [n, 1]and data stored in the memory circuits 82 [1].

Here, the value of biological data acquired by the circuit 20 (see FIGS.1A and 1B) is stored in the memory circuits 81, and a predeterminedreference value is stored in the memory circuits 82. The circuit 91 cancompare the value of m-bit biological data stored in memory circuits 81in a specific row and the value of m-bit reference value stored in thememory circuits 82. Accordingly, whether the acquired biological valueis normal or abnormal can be determined. For example, when the value ofthe biological data stored in the memory circuits 81 is greater than orequal to the reference value stored in the memory circuits 82, it isdetermined that the acquired biological data is abnormal.

The circuit 92 has a function of outputting an interrupt signal to thecircuit 60 (see FIG. 1) when the biological data is determined to beabnormal as a result of the comparison in the circuit 91. For example,the circuit 92 outputs data “1” when the biological data is normal, andoutputs data “0” as an interrupt signal when the biological data isabnormal. When the data “0” is output to the circuit 60, the circuit 70is controlled by the circuit 60, and a signal indicating that thebiological data is abnormal is transmitted from the circuit 70 to theoutside.

Note that the plurality of circuits 83 may be provided in the same layeras the circuit 90 (over the substrate 100 in FIG. 1C, and FIGS. 2A and2B), or in the same layer as the circuit 80 (over the insulating layer101 in FIG. 1C, and FIGS. 2A and 2B). In the case of using OStransistors for the circuits 83, the circuits 83 are preferably formedin the same layer as the circuit 80. In this case, the OS transistorsincluded in the circuits 83 and the OS transistors included in thememory circuits 81 and the memory circuits 82 can be manufacturedthrough the same process.

FIGS. 6A to 6C illustrate specific structural examples of the memorycircuit 81 and the memory circuit 82.

FIG. 6A illustrates a structural example of the memory circuit 81. Thememory circuit 81 includes a transistor 201 and a capacitor 202. A gateof the transistor 201 is connected to the wiring 111, one of a sourceand a drain thereof is connected to the wiring 121, and the other of thesource and the drain is connected to a node M1. One electrode of thecapacitor 202 is connected to the node M1, and the other electrode ofthe capacitor 202 is connected to the wiring 203. Note that withoutlimitation to the case where the transistor 201 is an n-channeltransistor, the transistor 201 may be either an n-channel transistor ora p-channel transistor. The wiring 203 may be either a high potentialpower supply line or a low potential power supply line (such as a groundline). Biological data can be stored in the memory circuit 81.

Thus, an OS transistor is used as the transistor 201. The transistormarked with a symbol “OS” in the drawing is an OS transistor (the sameapplies hereafter). The OS transistor has extremely low off-statecurrent; thus, the potential of the node M1 can be held for a long timein a period where the transistor 201 is turned off. Thus, the memorycircuit 81 can be used as a nonvolatile memory cell or a memory cellwith an extremely low refresh frequency.

The OS transistor can operate at a high speed when miniaturized. Thus,the operation speed of the memory circuit 81 can be improved by the useof the OS transistor as the transistor 201.

Next, operation of the memory circuit 81 illustrated in FIG. 6A isdescribed.

First, writing potential is supplied to the wiring 121. After thepotential of the wiring 203 is kept at a constant potential, thepotential of the wiring 111 is set to a potential at which thetransistor 201 is turned on, so that the transistor 201 is turned on.Accordingly, the potential of the wiring 121 is supplied to the node M1(data writing).

Next, the potential of the wiring 111 is set to a potential at which thetransistor 201 is turned off, so that the transistor 201 is turned off.This makes the node M1 floating, and the potential of the node M1 isheld (data holding). Since the transistor 201 is an OS transistor withextremely small off-state current, the potential of the node M1 can beheld for a long time.

After the wiring 121 is set in a floating state and the potential of thewiring 203 is kept at a constant potential, the potential of the wiring111 is set to a potential at which the transistor 201 is turned on, sothat the transistor 201 is turned on. Accordingly, the potential of thenode M1 is supplied to the wiring 121. At this time, the potential ofthe wiring 121 varies depending on the potential of the node M1. Datastored in the memory circuit 81 can be read out by reading the potentialof the wiring 121 at this time.

Rewriting of data can be performed in a manner similar to that of thewriting and holding of the data.

FIG. 6B illustrates a structural example of the memory circuit 82. Thememory circuit 82 includes a transistor 211, a capacitor 212, and acircuit 214. A gate of the transistor 211 is connected to the wiring112, one of a source and a drain thereof is connected to the wiring 121,and the other of the source and the drain thereof is connected to a nodeM2. One electrode of the capacitor is connected to the node M2 and theother electrode thereof is connected to a wiring 213 to which apredetermined potential is supplied. An input terminal of the circuit214 is connected to the node M2, and an output terminal thereof isconnected to the circuit 90. Note that the transistor 211 is an OStransistor. Without limitation to the case where the transistor 211 isan n-channel transistor, the transistor 211 may be either an n-channeltransistor or a p-channel transistor. The wiring 213 may be either ahigh potential power supply line or a low potential power supply line(such as a ground line).

In the memory circuit 82, data writing, data holding, and data rewritingcan be performed in a manner similar to that in the memory circuit 81 inFIG. 6A. In the memory circuit 82, a reference value for being comparedwith the value of the biological data stored in the memory circuit 81can be stored.

Furthermore, the memory circuit 82 can output data corresponding to apotential held in the node M2 to the circuit 90 through the circuit 214.Here, the circuit 214 is not specifically limited as long as it has afunction of outputting a signal corresponding to the potential of thenode M2 while keeping the potential of the node M2. For the circuit 214,a logic element such as an inverter or an analog switch can be used, forexample. In the case of using an inverter for the circuit 214, an inputterminal of the inverter is connected to the node M2, and an outputterminal thereof is connected to the circuit 90. For the arithmeticoperation in the circuit 90, an inverted signal of a signal output fromthe output terminal of the inverter can be used.

As illustrated in FIG. 6C, the memory circuit 82 may include atransistor 215. A gate of the transistor 215 is connected to a wiring216, one of a source and a drain thereof is connected to the node M2,and the other of the source and the drain thereof is connected to theinput terminal of the circuit 214. Note that the transistor 215 is an OStransistor.

The wiring 216 is a wiring to which a signal for turning on thetransistor 215 is supplied when a comparison operation is performed inthe circuit 90. Thus, a signal synchronized with a signal supplied tothe wiring 113 in FIG. 5 can be supplied to the wiring 216. For example,the wiring 216 may be connected to the wiring 113; alternatively, thewiring 113 may be directly connected to the gate of the transistor 215.Further alternatively, an inverted signal of the wiring 113 may besupplied to the wiring 216.

When the comparison operation is performed in the circuit 90, thetransistor 215 is turned on. In a period during which the comparisonoperation is not performed in the circuit 90, the transistor 215 is off.Here, since the transistor 215, which is an OS transistor, has extremelysmall off-state current, the potential of the node M2 can be preventedfrom leaking to the circuit 90 through the circuit 214. Thus, thepotential held in the node M2 can be held for a long period.

Next, FIGS. 7A to 7C show another structural examples of the memorycircuit 81 and the memory circuit 82.

FIG. 7A illustrates another structural example of the memory circuit 81.The memory circuit 81 includes a transistor 221, a transistor 222, and acapacitor 223. A gate of the transistor 221 is connected to the wiring111, one of a source and a drain thereof is connected to the wiring 121,and the other of the source and the drain thereof is connected to a nodeM3. A gate of the transistor 222 is connected to the node M3, one of asource and a drain thereof is connected to the wiring 121, and the otherof the source and the drain thereof is connected to the wiring 122. Oneelectrode of the capacitor 223 is connected to the node M3 and the otherelectrode thereof is connected to a wiring 224 to which a predeterminedpotential is supplied. Here, an OS transistor is used as the transistor221. Note that the wiring 122 is connected to the circuit 120 (see FIG.5).

Note that without limitation to the case where the transistor 221 andthe transistor 222 are n-channel transistors, each of the transistor 221and the transistor 222 may be either an n-channel transistor or ap-channel transistor. The wiring 224 may be a high potential powersupply line or a low potential power supply line (such as a groundline).

For the transistor 222, a transistor including a single crystalsemiconductor in its channel formation region can be used. In that case,current supply capability of the transistor 222 can be improved,resulting in high-speed operation of the memory circuit 81. For thetransistor 222, an OS transistor can be used. In that case, thetransistor 222 and the transistor 221 can be formed in the same process.

Next, operation of the memory circuit 81 illustrated in FIG. 7A will bedescribed.

After the potential of the wiring 111 is set to a potential at which thetransistor 221 is turned on, the transistor 221 is turned on.Accordingly, the potential of the wiring 121 is supplied to the node M3.That is, a predetermined charge is supplied to the gate electrode of thetransistor 222 (data writing).

After that, the potential of the wiring 111 is set to a potential atwhich the transistor 221 is turned off, so that the transistor 221 isturned off. This makes the node M3 floating, and the potential of thenode M3 is held (data holding).

After the potential of the wiring 122 is kept at a constant potential,the potential of the wiring 224 is set to a predetermined potential (areading potential), so that the potential of the wiring 121 variesdepending on the amount of the charge held in the node M3. This isbecause in general, in the case where the transistor 222 is an n-channeltransistor, an apparent threshold voltage V_(th) _(_) _(H) when thepotential of the gate of the transistor 222 is high is lower than anapparent threshold voltage V_(th) _(_) _(L) when the potential of thegate of the transistor 222 is low. Here, an apparent threshold voltagerefers to the potential of the wiring 224 that is needed to turn on thetransistor 222. Thus, by setting the potential of the wiring 224 to apotential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_) _(L), thepotential of the node M3 can be determined. For example, in the casewhere the potential of the node M3 is a high level, the transistor 222is turned on when the potential of the wiring 224 becomes V₀(>V_(th)_(_) _(H)). In the case where the potential of the node M3 is a lowlevel, the transistor 222 remains off even when the potential of thewiring becomes V₀(>V_(th) _(_) _(L)). Thus, the data stored in thememory circuit 81 can be read out by reading out the potential of thewiring 121.

In the case where data reading is not performed, a potential at whichthe transistor 222 is turned off regardless of the potential of the nodeM3, that is, a potential lower than V_(th) _(_) _(H) is supplied to thewiring 224.

Rewriting of data can be performed in a manner similar to that of thewriting and holding of the data.

Note that one of the source and the drain of the transistor 221 iselectrically connected to the gate of the transistor 222, and therebyhas an effect similar to that of a floating gate of a floating-gatetransistor which is used as a non-volatile memory element. Thus, thenode M3 is referred to as a floating gate portion FG in some cases. Whenthe transistor 221 is off, the floating gate portion FG can be regardedas being embedded in an insulator and charge is held in the floatinggate portion FG. The off-state current of the transistor 221 is lessthan or equal to 1/100,000 of the off-state current of a transistorincluding a single crystal semiconductor in its channel formationregion; thus, loss of the charge accumulated in the floating gateportion FG due to leakage current of the transistor 221 is extremelysmall. Alternatively, loss of the charge accumulated in the floatinggate portion FG is negligible for a long period. As a result, with theuse of the transistor 221 that is an OS transistor, a nonvolatile memorydevice or a memory device capable of retaining data for a long periodwithout power supply can be realized.

In the memory circuit 81, data can be directly rewritten by anotherwriting of data. For that reason, erasing operation which is needed in aflash memory or the like is not needed, whereby a reduction in operationspeed caused by erasing operation can be suppressed. That is, high-speedoperation of the semiconductor device can be realized.

Further, in that case, the problem of deterioration of a gate insulatingfilm (tunnel insulating film), which is pointed out in a conventionalfloating gate transistor, does not exist. That is to say, thedeterioration of a gate insulating film due to injection of an electroninto a floating gate, which has been traditionally regarded as aproblem, can be neglected. This means that there is no limit on thenumber of times of writing in principle. Furthermore, a high voltageneeded for writing or erasing in a conventional floating gate transistoris not necessary.

The OS transistor can operate at a high speed when miniaturized. Thus,the operation speed of the memory circuit 81 can be improved by the useof the OS transistor as the transistor 201.

FIG. 7B illustrates another structural example of the memory circuit 82.The memory circuit 82 includes a transistor 231, a transistor 232, acapacitor 233, and a transistor 234. A gate of the transistor 231 isconnected to the wiring 112, one of a source and a drain thereof isconnected to the wiring 121, and the other of the source and the drainthereof is connected to a node M4. A gate of the transistor 232 isconnected to the M4, one of a source and a drain thereof is connected tothe wiring 122, and the other of the source and the drain thereof isconnected to one of a source and a drain of the transistor 234. Oneelectrode of the capacitor 233 is connected to the node M4 and the otherelectrode thereof is connected to the wiring 122. A gate of thetransistor 234 is connected to the wiring 235, and the other of thesource and the drain thereof is connected to a node M5. Thus, an OStransistor is used as the transistor 231. Note that the wiring 122 isconnected to the circuit 120 (see FIG. 5)

Operation of the memory circuit 82 illustrated in FIG. 7B will bedescribed.

First, the potential of the wiring 112 is set to a potential at whichthe transistor 231 is turned on, so that the transistor 231 is turnedon. Accordingly, the potential of the wiring 121 is supplied to the nodeM4. That is, a predetermined potential is supplied to the gate electrodeof the transistor 232 (data writing).

After that, the potential of the wiring 112 is set to a potential atwhich the transistor 231 is turned off, so that the transistor 231 isturned off. This makes the node M4 floating, and the potential of thenode M4 is held (data holding)

After that, a potential at which the transistor 234 is turned on(hereinafter also referred to as a reading potential) is supplied to thewiring 235 while a constant potential is supplied to the wiring 122, sothat the transistor 234 is turned on. At this time, the potential of thenode M5 varies depending on the amount of charge held in the node M4.This is because the transistor 234 is turned on in the case where thepotential of the node M4 is high and is turned off in the case where thepotential of the node M4 is low. As described above, the potential ofthe node M5 depending on the potential of the node M4 is supplied to thecircuit 90.

Note that the structure of the memory circuit 82 that is changed byconnecting the node M5 to the wiring 121 can be used as the structure ofthe memory circuit 81.

A reading potential is supplied to the wiring 235 when a comparisonoperation is performed in the circuit 90. This reading potential can besynchronized with the potential of the wiring 113 in FIG. 5. Forexample, the wiring 235 may be connected to the wiring 113, or thewiring 113 may be directly connected to the gate of the transistor 234.Furthermore, an inverted signal of the wiring 113 may be supplied to thewiring 235.

When the comparison operation is performed in the circuit 90, a readingpotential is supplied to the wiring 235, so that the transistor 234 isturned on. When the transistor 234 is turned on, a potentialcorresponding to the potential of the node M4 is supplied from the nodeM5 to the circuit 90. In a period during which the comparison operationis not performed in the circuit 90, a potential at which the transistor234 is turned off is supplied to the wiring 235.

The memory circuit 82 may have a structure shown in FIG. 7C. FIG. 7C isdifferent from FIG. 7B in that the memory circuit 82 includes atransistor 236.

A gate of the transistor 236 is connected to a wiring 237, one of asource and a drain thereof is connected to a node M6, and the other ofthe source and the drain thereof is connected to the wiring 121.

A potential similar to the potential that is supplied to the wiring 235in FIG. 7B is supplied to the wiring 235. A potential for controllingon/off of the transistor 236 is supplied to the wiring 237. Accordingly,the data held in the memory circuit 82 can be output not only to thecircuit 90 but also to the wiring 121. Then, by reading the potential ofthe wiring 121 at the time of turning on the transistor 236, data storedin the memory circuit 82 can be read out.

With any of the above-described structures, the reference value ofbiological data stored in the memory circuit 82 can be held for a longperiod even in a period during which supply of power to the memorycircuit 82 is stopped. Thus, after the reference value is written to thememory circuit 82 once, the reference value can be held for a longperiod even in a period during which supply of power to the memorycircuit 82 is stopped. When the comparison operation is performed in thecircuit 90, the reference value stored in the memory circuit 82 can beoutput to the circuit 90.

Note that the circuit 80 may include the memory circuit 81 illustratedin FIG. 6A and the memory circuit 82 illustrated in FIG. 7B.Alternatively, the circuit 80 may include the memory circuit 81illustrated in FIG. 7A and the memory circuit 82 illustrated in FIG. 6Bor 6C.

Next, a specific structure example of the circuit 90 is described.

FIG. 8 illustrates a specific structure example of the circuit 90. Here,a description is made on a structure of the circuit 90 which has afunction of comparing two input data.

The circuit 90 includes the circuit 91 and the circuit 92. The circuit91 includes an XNOR circuit 301 and a NOR circuit 302. A first inputterminal of the XNOR circuit 301 is connected to the circuit 83, and asecond input terminal thereof is connected to the memory circuit 82. Afirst input terminal of the NOR circuit 302 is connected to the circuit83, and a second input terminal thereof is connected to an outputterminal of the XNOR circuit 301. The output terminals of the XNORcircuit 301 and the NOR circuit are connected to the circuit 92.

The circuit 91 includes a comparison circuit. Thus, the value ofbiological data input from the memory circuit 81 through the circuit 83and the reference value stored in the memory circuit 82 can be comparedwith each other, and the result can be output to the circuit 92.

The circuit 92 includes an inverter 303 and an AND circuit 304. An inputterminal of the inverter 303 is connected to the output terminal of theXNOR circuit 301. A first input terminal of the AND circuit 304 isconnected to the output terminal of the NOR circuit 302, and a secondinput terminal thereof is connected to an output terminal of theinverter 303.

As a result of the comparison between the value of the biological datain the circuit 91 and the reference value, the circuit 92 outputs data“1” to the circuit 60 when the value of the biological data is less thanthe reference value, and outputs data “0” to the circuit 60 as aninterrupt signal when the value of the biological data is greater thanor equal to the reference value. When the interrupt signal is input tothe circuit 60, the circuit 70 is controlled by the circuit 60, and asignal indicating that the biological data is abnormal is transmittedfrom the circuit 70 to the outside.

As described above, the circuit 90 determines whether the biologicaldata is normal or abnormal and outputs the determination result to thecircuit 60.

FIG. 9 illustrates a more specific structure of the circuit 50. Notethat the memory circuit 82 in FIG. 9 corresponds to the structureillustrated in FIG. 6B, and the circuits 91 and 92 in FIG. 9 correspondto the structure illustrated in FIG. 8. An n-channel transistor is usedas the circuit 83, and an inverter is used as the circuit 214. Althoughnot shown in the figure, the structure illustrated in FIG. 6A or thelike can be used for the memory circuit 81 connected to the wiring 121.

As illustrated in FIG. 9, each of the memory circuit 82, the circuit 91,and the circuit 92 includes an n-channel transistor and a p-channeltransistor.

Here, transistors 312, 322, 324, 333, 334, 342, 351, and 352 which aren-channel transistors are OS transistors. Transistors 311, 321, 323,331, 332, 341, 353, and 354 which are p-channel transistors aretransistors including a single crystal semiconductor in their channelformation regions. With such a structure, the n-channel transistorsincluded in the circuit 50 can be formed in the same process as thetransistor 211 that is an OS transistor. In manufacturing the transistor50, it is not necessary to form the n-channel transistors including asingle crystal semiconductor in their channel formation regions; thus,the number of manufacturing steps can be reduced.

Here, in each of FIGS. 1A to 1C, FIGS. 2A and 2B, and FIGS. 3A to 3D,the circuit 50 has a structure in which the circuit 90 and the circuit80 are stacked; however, the circuit 50 may have a structure in which ap-channel transistor and an n-channel transistor are stacked.Specifically, the transistors 311, 321, 323, 331, 332, 341, 353, and 354that are p-channel transistors can be transistors whose channelformation regions can be formed in part of the substrate 100 in FIG. 1C,FIGS. 2A and 2B, and FIGS. 3A to 3D. The transistors 211, 312, 322, 324,333, 334, 342, 351, and 352 that are n-channel transistors are OStransistors and can be formed over the insulating layer 101 (see FIG.1C, FIGS. 2A and 2B) that is provided over the p-channel transistors.Thus, the area of the circuit 50 can be reduced, and manufacturing of ann-channel transistor having a single crystal semiconductor in itschannel formation region can be omitted.

Note that description is made on a structure in which the circuit 90includes a one-bit comparison circuit in FIG. 8 and FIG. 9. Whenmulti-bit data is compared with multi-bit data in the circuit 90, thecircuit 90 may have a comparison circuit for multi-bit data. FIG. 10illustrates an example in which the circuit 90 has a comparison circuitusing four-bit data as an input signal.

The circuit 90 includes inverters 401 to 405, XOR circuits 411 to 413,AND circuits 421 to 424, and NOR circuits 431 and 432. The connectionrelation of the circuits is clear from FIG. 10; thus, detaileddescription is omitted.

Here, data stored in four memory circuits 81 of the plurality of memorycircuits 81 in the same row (see FIG. 5) is input to wirings A asfour-bit biological data, and data stored in four memory circuits 82 inthe plurality of memory circuits 82 is input to wirings B as a four-bitreference value.

Then, the value of the biological data and the reference value arecompared with each other in the circuit 90. When the value of thebiological data is less than the reference value, data “0” is outputfrom a wiring C, and when the value of the biological data is greaterthan or equal to the reference value, data “1” is output from the wiringC. As described above, in the circuit 90 illustrated in FIG. 10,multi-bit biological data can be compared with multi-bit referencevalue.

In FIG. 8, FIG. 9, and FIG. 10, description is made on an example inwhich the circuit 90 has the comparison circuit; however, the structureof the present invention is not limited to this. For example, thecircuit 90 may have another arithmetic circuit instead of or in additionto the comparison circuit. FIGS. 11A and 11B illustrate examples ofanother arithmetic circuit that can be used as the circuit 90.

FIG. 11A illustrates an adder circuit including an XOR circuit 501 andan AND circuit 502. FIG. 11B is a subtractor circuit including inverters511 and 512, AND circuits 513 and 514, and an OR circuit 515. Thecircuit 90 may have a full adder circuit or a full subtractor circuitformed by a combination of an adder circuit and a subtractor circuitillustrated in FIG. 11. In addition, the circuit 90 may have a dividercircuit formed using the full adder circuit or the full subtractorcircuit.

The circuit 90 includes the adder circuit and the divider circuit; thus,an average value of the biological data stored in the memory circuit 81can be calculated. Furthermore, the circuit 90 includes the subtractorcircuit, a difference in biological data stored in the memory circuit 81can be calculated and variation in the biological data can be monitored.

In each of FIGS. 8, 9, 10, and 11A and 11B, description is made on thecase where the circuit 90 includes a digital arithmetic circuit;however, the circuit 90 may include an analog arithmetic circuit. FIGS.12A to 12D each illustrate a structural example of an analog arithmeticcircuit using an operational amplifier 520, which can be used for thecircuit 90.

FIG. 12A is a comparison circuit. FIG. 12B is an adder circuit. FIG. 12Cis a subtractor circuit. FIG. 12D is a divider circuit. Note that inFIG. 12D, the resistance value of a resistor R is controlled by thepotential V.

As described above, in one embodiment of the present invention, thecircuit 50 can be used as a memory circuit which has a function ofperforming an arithmetic operation. Thus, the circuit 50 can output tothe circuit 60 data stored in the circuit 50, data input from thecircuit 40, and data obtained as the result of an arithmetic operationusing aforementioned data as an input signal. Thus, the circuit 50 canperform the arithmetic operation that is essentially performed in thecircuit 60, accordingly, a burden of the arithmetic operation on thecircuit 60 can be reduced. Furthermore, the number of times of datatransmission and reception between the circuits 50 and 60 can bereduced. As a result, the operation speed of the semiconductor device 10can be improved.

Further, in one embodiment of the present invention, a structure can beemployed in which one of the circuit 80 and the circuit 90 has a regionoverlapping with at least part of the other of the circuit 80 and thecircuit 90. Thus, the circuit 50 which functions as a memory circuit canhave an additional function of performing an arithmetic operation whilethe increase in the area of the circuit 50 is suppressed. Thus, the areaof the semiconductor device 10 can be reduced.

This embodiment can be combined with any other embodiment asappropriate.

Embodiment 3

In this embodiment, the structure of a transistor that can be used inthe circuit 50 will be described.

FIGS. 13A to 13D illustrates an example of a method for manufacturing asemiconductor device in which a transistor 620 and a transistor 630 arestacked. Here, the transistor 620 is a transistor including a singlecrystal semiconductor in its channel formation region and the transistor630 is an OS transistor.

First, an element isolation insulator 601 and an n-type well 602 areformed in a semiconductor substrate 600 (FIG. 13A).

Next, a gate insulating film 603 and a gate electrode 604 are formed,and a p-type impurity region 605 is formed in the well 602. A layerwhich includes a higher conductivity material (such as silicide) thanthe impurity region 605 may be stacked over the impurity region 605. Theimpurity region 605 may include an extension region.

Next, an insulating layer 606 is formed. The insulating layer 606 may bea single layer or a multilayer and is preferably capable of supplyingoxygen to a layer provided over the insulating layer 606 and blockingthe entry of hydrogen or water from a layer provided below theinsulating layer 606 to the layer provided over the insulating layer606. The insulating layer 606 is etched and planarized. The etching andthe planarizing are stopped when the gate electrode 604 is exposed. Notethat the planarizing of the insulating layer 606 can be performed bychemical mechanical polishing (CMP).

Next, an oxide semiconductor layer 607 is formed over the insulatinglayer 606 (FIG. 13B). The oxide semiconductor layer 607 can be formedusing a material described in Embodiment 4.

Next, a conductive film is formed over the insulating layer 606 and theoxide semiconductor layer 607. The conductive film may be a single layeror a multilayer. Then, the conductive film is etched so as to form aconductive layer 608. The conductive layer 608 has a function as asource electrode or a drain electrode of a transistor which has achannel formation region in the oxide semiconductor layer 607. Theconductive layer 608 may be a single layer or a multilayer.

Next, a gate insulating layer 609 is formed to cover the conductivelayer 608. Further, a conductive film is formed over the gate insulatingfilm 609. The conductive film may be a single layer or a multilayer. Theconductive film is preferably capable of blocking the entry of hydrogenor water from a layer provided over the conductive film to the layerprovided below the conductive film. Then, the conductive film is etchedso that the gate electrode 610 is formed (FIG. 13C).

Then, an insulating layer 611 is formed. A contact hole reaching theconductive layer 608 is formed in the insulating layer 611, and then isfilled with a conductive material, whereby a wiring 612 is formed (FIG.13D). Note that a conductive layer in contact with the conductive layer608 may be formed in the contact hole, so that the conductive layer andthe wiring 612 may be in contact with each other. Note that the wiring612 may be a single layer or a multilayer.

As described-above, the semiconductor device in which the transistor 620including a single crystal semiconductor in its channel formation regionand the transistor 630 that is an OS transistor are stacked can bemanufactured.

Note that in FIG. 13D, the gate electrode 604 is connected to theconductive layer 608. That is, a gate of the transistor 620 is connectedto one of a source and a drain of the transistor 630. Such a structurecan be applied to the circuits in FIGS. 7A to 7C and FIG. 9 asappropriate. For example, the transistor 620 corresponds to thetransistors 222, 232, and the like in FIGS. 7A to 7C, and the transistor630 corresponds to the transistors 221, 231, and the like in FIGS. 7A to7C. Further, the transistor 620 corresponds to the transistor 321 andthe like in FIG. 9, and the transistor 630 corresponds to the circuit(transistor) 83 and the like in FIG. 9.

Note that the connection relation between the transistor 620 and thetransistor 630 is not limited to that shown in FIG. 13D. For example, asillustrated in FIG. 14A, the impurity region 605 may be connected to thegate electrode 610 through the wiring 612. Accordingly, one of a sourceand a drain of the transistor 620 may be connected to the gate of thetransistor 630. Such a structure can be appropriately used in thecircuit illustrated in FIG. 9 and the like. For example, the transistor620 corresponds to the transistors 311, 332, and the like in FIG. 9, andthe transistor 630 corresponds to the transistor 324, 352, and the likein FIG. 9.

As shown in FIG. 14B, the impurity region 605 may be connected to theconductive layer 608. With this structure, one of a source and a drainof the transistor 620 can be connected to one of a source and a drain ofthe transistor 630. Such a structure can be appropriately used in thecircuit shown in FIG. 9, and the like. For example, the transistor 620corresponds to the transistor 311 in FIG. 9 and the like, and thetransistor 630 corresponds to the transistor 312 in FIG. 9 and the like.

As shown in FIG. 14C, the gate electrode 604 can be connected to thegate electrode 610 through the wiring 612. Accordingly, the gate of thetransistor 620 can be connected to the gate of the transistor 630. Sucha structure can be appropriately used for the circuit illustrated inFIG. 9 and the like. For example, the transistor 620 corresponds to thetransistor 311 illustrated in FIG. 9 and the like, and the transistor630 corresponds to the transistor 312 illustrated in FIG. 9 and thelike. Such a structure is effective particularly in the case where aninverter is formed using an OS transistor and a transistor including asingle crystal semiconductor in its channel formation region.

In each of FIG. 13D and FIGS. 14A to 14C, the transistor 620 and thetransistor 630 may have mutually overlapping regions with the insulatinglayer 606 therebetween. For example, as illustrated in FIG. 13D and FIG.14C, the impurity region 605 of the transistor 620 and the channelformation region of the transistor 630 may have mutually overlappingregions with the insulating layer 606 therebetween. Furthermore, asillustrated in FIGS. 14A and 14B, the channel formation region of thetransistor 620 and the channel formation region of the transistor 630may have mutually overlapping regions with the insulating layer 606therebetween. Furthermore, the gate electrode 604 of the transistor 620and the gate electrode 610 of the transistor 630 may have mutuallyoverlapping regions with the insulating layer 606 therebetween. With anyof these structures, the integration degree of the transistors can beimproved.

The stacked-layer structure of the transistor illustrated in any of FIG.13D, and FIGS. 14A to 14C can be freely used in any of the circuitsshown in FIGS. 1A to 1C, FIGS. 2A and 2B, FIGS. 3A to 3D, FIG. 5, FIGS.6A to 6C, FIGS. 7A to 7C, FIG. 8, FIG. 9, FIG. 10, FIGS. 11A and 11B,and FIGS. 12A to 12D.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and the like.

Embodiment 4

In this embodiment, the structures of a transistor that can be used inthe memory circuit or a logic circuit will be described.

<Example of Cross-Sectional Structure of Semiconductor Device>

FIG. 15 illustrates a structure example of the transistors 620 and 630.In FIG. 15, the transistor 630 that is an OS transistor is formed overthe transistor 620 that is a transistor including a material other thanan oxide semiconductor in its channel formation region.

Note that such a structure in which the transistor including a materialother than an oxide semiconductor and the OS transistor are stacked canbe appropriately used for any of transistors included in the variouscircuit illustrated in FIGS. 1A to 1C, FIGS. 2A and 2B, FIGS. 3A to 3D,FIG. 5, FIGS. 6A to 6C, FIGS. 7A to 7C, FIG. 8, FIG. 9, FIG. 10, FIGS.11A and 11B, and FIGS. 12A to 12D.

In this embodiment, as in FIG. 13D, the gate of the transistor 620 isconnected to one of the source and the drain of the transistor 630;however, the structure of the present invention is not limited to this.One of the source and the drain of the transistor 620 may be connectedto the gate of the transistor 630 (see FIG. 14A), one of the source andthe drain of the transistor 620 may be connected to one of the sourceand the drain of the transistor 630 (see FIG. 14B), or the gate of thetransistor 620 may be connected to the gate of the transistor 630 (seeFIG. 14C).

The transistor 620 may include its channel formation region in asemiconductor film or a semiconductor substrate of silicon, germanium,or the like in an amorphous, microcrystalline, polycrystalline, orsingle crystal state. Alternatively, the transistor 620 may include itschannel formation region in an oxide semiconductor film or an oxidesemiconductor substrate. In the case where channel formation regions ofall the transistors are included in an oxide semiconductor film or anoxide semiconductor substrate, the transistor 630 is not necessarilystacked over the transistor 620, and the transistors 630 and 620 may beformed in the same layer.

In the case where the transistor 620 is formed using a thin siliconfilm, any of the following can be used in the thin film: amorphoussilicon formed by a sputtering method or a vapor phase growth methodsuch as a plasma-enhanced chemical vapor deposition (CVD) method;polycrystalline silicon obtained by crystallization of amorphous siliconby treatment such as laser annealing; single crystal silicon obtained byseparation of a surface portion of a single crystal silicon wafer byimplantation of hydrogen ions or the like into the silicon wafer; andthe like.

A semiconductor substrate 801 where the transistor 620 is formed can be,for example, a silicon substrate, a germanium substrate, or a silicongermanium substrate. FIG. 15 shows the case where a single crystalsilicon substrate is used as the semiconductor substrate 801.

The transistor 620 is electrically isolated by element isolation. As theelement isolation method, a local oxidation of silicon (LOCOS) method, ashallow trench isolation (STI) method, or the like can be employed.FIGS. 14A to 14C illustrate an example where the trench isolation methodis used to electrically isolate the transistor 620. Specifically, inFIG. 15, the transistor 620 is electrically isolated by elementisolation using an element isolation region 810 formed in such a mannerthat an insulator including silicon oxide or the like is buried in atrench formed in the semiconductor substrate 801 by etching or the like.

An insulating film 811 is provided over the transistor 620. Openingportions are formed in the insulating film 811. Conductive films 825 and826 that are electrically connected to the source and the drain of thetransistor 620 and a conductive film 827 that is electrically connectedto the gate of the transistor 620 are formed in the openings.

The conductive film 825 is electrically connected to a conductive film834 formed over the insulating film 811. The conductive film 826 iselectrically connected to a conductive film 835 formed over theinsulating film 811. The conductive film 827 is electrically connectedto a conductive film 836 formed over the insulating film 811.

An insulating film 812 is formed over the conductive films 834 to 836.An opening is formed in the insulating film 812. A conductive film 837electrically connected to the conductive film 836 is formed in theopening portion. The conductive film 837 is electrically connected to aconductive film 851 formed over the insulating film 812.

An insulating film 813 is formed over the conductive film 851. Anopening is formed in the insulating film 813. A conductive film 852electrically connected to the conductive film 851 is formed in theopening. The conductive film 852 is electrically connected to aconductive film 853 formed over the insulating film 813. A conductivefilm 844 is formed over the insulating film 813.

An insulating film 861 is formed over the conductive film 853 and theconductive film 844. In FIG. 15, the transistor 630 is formed over theinsulating film 861.

The transistor 630 includes, over the insulating film 861, asemiconductor film 901 including an oxide semiconductor, conductivefilms 921 and 922 functioning as source and drain over the semiconductorfilm 901, a gate insulating film 862 over the semiconductor film 901 andthe conductive films 921 and 922, and a gate electrode 931 overlappingwith the semiconductor film 901 over the gate insulating film 862 andbetween the conductive films 921 and 922. Note that the conductive film922 is electrically connected to the conductive film 853 in an openingprovided in the insulating film 861.

In the semiconductor film 901 of the transistor 630, there is a region910 between a region overlapping with the conductive film 921 and aregion overlapping with the gate electrode 931. In addition, in thesemiconductor film 901 of the transistor 630, there is a region 911between a region overlapping with the conductive film 922 and the regionoverlapping with the gate electrode 931. When argon, an impurity whichimparts p-type conductivity to the semiconductor film 901, or animpurity which imparts n-type conductivity to the semiconductor film 901is added to the regions 910 and 911 using the conductive films 921 and922 and the gate electrode 931 as masks, the resistivity of the regions910 and 911 can be made lower than that of the region overlapping withthe gate electrode 931 in the semiconductor film 901.

An insulating film 863 is provided over the transistor 630.

In FIG. 15, the transistor 630 has the gate electrode 931 on at leastone side of the semiconductor film 901; alternatively, the transistor630 may have a pair of gate electrodes with the semiconductor film 901positioned therebetween.

In the case where the transistor 630 has a pair of gate electrodes withthe semiconductor film 901 positioned therebetween, one of the gateelectrodes may be supplied with a signal for controlling the on/offstate, and the other of the gate electrodes may be supplied with apotential from another wiring. In this case, potentials at the samelevel may be supplied to the pair of gate electrodes, or a fixedpotential such as the ground potential may be supplied only to the otherof the gate electrodes. By controlling the level of a potential suppliedto the other of the gate electrodes, the threshold voltage of thetransistor can be controlled.

In FIG. 15, the transistor 630 has a single-gate structure where onechannel formation region corresponding to one gate electrode 931 isprovided. However, the transistor 630 may have a multi-gate structurewhere a plurality of channel formation regions are formed in one activelayer by providing a plurality of gate electrodes electrically connectedto each other.

<Transistor>

Then, structure examples of the OS transistor will be described.

FIGS. 16A to 16C illustrate an example of a transistor 2000 that is anOS transistor. FIG. 16A is a top view of the transistor 2000. Note thatinsulating films are not illustrated in FIG. 16A in order to clarify thelayout of the transistor 2000. FIG. 16B is a cross-sectional view alongthe dashed-dotted line A1-A2 in the top view in FIG. 16A. FIG. 16C is across-sectional view along the dashed-dotted line A3-A4 in the top viewin FIG. 16A.

As illustrated in FIGS. 16A to 16C, the transistor 2000 includes anoxide semiconductor film 2002 a and an oxide semiconductor film 2002 bthat are stacked in this order over an insulating film 2001 formed overa substrate 2007; a conductive film 2003 and a conductive film 2004 thatare electrically connected to the oxide semiconductor film 2002 b andfunction as a source electrode and a drain electrode; an oxidesemiconductor film 2002 c over the oxide semiconductor film 2002 b, theconductive film 2003, and the conductive film 2004; an insulating film2005 that functions as a gate insulating film and is located over theoxide semiconductor film 2002 c; and a conductive film 2006 thatfunctions as a gate electrode, lies over the insulating film 2005, andoverlaps with the oxide semiconductor films 2002 a to 2002 c. Note thatthe substrate 2007 may be a glass substrate, a semiconductor substrate,or the like or may be an element substrate where semiconductor elementsare formed over a glass substrate or on a semiconductor substrate.

FIGS. 17A to 17C illustrate another specific example of the structure ofthe transistor 2000. FIG. 17A is a top view of the transistor 2000. Notethat insulating films are not illustrated in FIG. 17A in order toclarify the layout of the transistor 2000. FIG. 17B is a cross-sectionalview along the dashed-dotted line A1-A2 in the top view in FIG. 17A.FIG. 17C is a cross-sectional view along the dashed-dotted line A3-A4 inthe top view in FIG. 17A.

As illustrated in FIGS. 17A to 17C, the transistor 2000 includes theoxide semiconductor films 2002 a to 2002 c that are stacked in thisorder over the insulating film 2001; the conductive films 2003 and 2004that are electrically connected to the oxide semiconductor film 2002 cand function as a source electrode and a drain electrode; the insulatingfilm 2005 that functions as a gate insulating film and is located overthe oxide semiconductor film 2002 c and the conductive films 2003 and2004; and a conductive film 2006 that functions as a gate electrode,lies over the insulating film 2005, and overlaps with the oxidesemiconductor films 2002 a to 2002 c.

FIGS. 16A to 16C and FIGS. 17A to 17C each illustrate the structuralexample of the transistor 2000 in which the oxide semiconductor films2002 a to 2002 c are stacked. However, the structure of the oxidesemiconductor film included in the transistor 2000 is not limited to astacked-layer structure including a plurality of oxide semiconductorfilms and may be a single-layer structure.

In the case where the transistor 2000 includes the semiconductor film inwhich the semiconductor films 2002 a to 2002 c are stacked in thisorder, each of the oxide semiconductor films 2002 a and 2002 c is anoxide film that contains at least one of metal elements contained in theoxide semiconductor film 2002 b and in which energy at the conductionband minimum is closer to the vacuum level than that in the oxidesemiconductor film 2002 b by higher than or equal to 0.05 eV, 0.07 eV,0.1 eV, or 0.15 eV and lower than or equal to 2 eV, 1 eV, 0.5 eV, or 0.4eV. The oxide semiconductor film 2002 b preferably contains at leastindium because carrier mobility is increased.

When the transistor 2000 includes the semiconductor film with the abovestructure, when an electric field is applied to the semiconductor filmby applying voltage to the gate electrode, a channel region is formed inthe oxide semiconductor film 2002 b, which has the lowest conductionband energy among the oxide semiconductor films. That is, the oxidesemiconductor film 2002 c provided between the oxide semiconductor film2002 b and the insulating film 2005 makes it possible to form thechannel region in the oxide semiconductor film 2002 b, which isseparated from the insulating film 2005.

Since the oxide semiconductor film 2002 c contains at least one of themetal elements contained in the oxide semiconductor film 2002 b,interface scattering is unlikely to occur at the interface between theoxide semiconductor film 2002 b and the oxide semiconductor film 2002 c.Thus, the movement of carriers is unlikely to be inhibited at theinterface, which results in an increase in the field-effect mobility ofthe transistor 2000.

When an interface state is formed at an interface between the oxidesemiconductor films 2002 b and 2002 a, a channel region is also formedin a region close to the interface; thus, the threshold voltage of thetransistor 2000 varies. However, since the oxide semiconductor film 2002a contains at least one of metal elements contained in the oxidesemiconductor film 2002 b, an interface state is unlikely to be formedat the interface between the oxide semiconductor film 2002 b and theoxide semiconductor film 2002 a. As a result, such a structure canreduce variations in electrical characteristics (e.g., thresholdvoltage) of the transistor 2000.

Further, it is preferable that a plurality of oxide semiconductor filmsbe stacked so that an interface level due to an impurity existingbetween the oxide semiconductor films, which inhibits carrier flow, isnot formed at an interface between the oxide semiconductor films. Thisis because when an impurity exists between the stacked oxidesemiconductor films the continuity of the energies of the bottoms of theconduction bands of the oxide semiconductor films is lost, and carriersare trapped or disappear by recombination in the vicinity of theinterface. By reducing an impurity existing between the films, acontinuous junction (here, in particular, a well structure having a Ushape in which energies of the bottoms of the conduction bands arechanged continuously between the films) is formed easily as comparedwith the case of merely stacking the plurality of oxide semiconductorfilms which contain at least one common metal as a main component.

In order to form such a continuous energy band, it is necessary to formfilms continuously without being exposed to air, with use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber of the sputtering apparatus ispreferably evacuated to a high vacuum (to 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) by anadsorption vacuum pump such as a cryopump so that water and the likeacting as impurities for the oxide semiconductor are removed as much aspossible. Alternatively, a turbo molecular pump and a cold trap arepreferably used in combination to prevent backflow of gas into thechamber through an evacuation system.

To obtain a highly purified intrinsic oxide semiconductor, not only highvacuum evacuation of the chambers but also high purification of a gasused in the sputtering is important. When an oxygen gas or an argon gasused as the above gas has a dew point of −40° C. or lower, preferably−80° C. or lower, further preferably −100° C. or lower and is highlypurified, moisture and the like can be prevented from entering the oxidesemiconductor film as much as possible. In the case where the oxidesemiconductor film 2002 b is formed of an In-M-Zn oxide (M is Al, Ti,Ga, Y, Zr, La, Ce, Nd, or Hf) and a target having the atomic ratio ofmetal elements of In:M:Zn=x₁:y₁:z₁ is used for depositing the oxidesemiconductor layer 2002 b, x₁/y₁ is preferably greater than or equal to⅓ and less than or equal to 6, further preferably greater than or equalto 1 and less than or equal to 6, and z₁/y₁ is preferably greater thanor equal to ⅓ and less than or equal to 6, further preferably greaterthan or equal to 1 and less than or equal to 6. Note that when z₁/y₁ isgreater than or equal to 1 and less than or equal to 6, a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) film as the oxidesemiconductor film 2002 b is easily formed. Typical examples of theatomic ratio of the metal elements of the target are In:M:Zn=1:1:1 andIn:M:Zn=3:1:2. Note that the CAAC-OS will be described in detail later.

In the case where the oxide semiconductor films 2002 a and 2002 c eachcontain an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) anda target having the atomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂is used for forming the oxide semiconductor films 2002 a and 2002 c,x₂/y₂ is preferably less than x₁/y₁, and z₂/y₂ is preferably greaterthan or equal to ⅓ and less than or equal to 6, further preferablygreater than or equal to 1 and less than or equal to 6. Note that whenz₂/y₂ is greater than or equal to 1 and less than or equal to 6, CAAC-OSfilms are easily formed as the oxide semiconductor films 2002 a and 2002c. Typical examples of the atomic ratio of the metal elements of thetarget are In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8,and the like.

The oxide semiconductor film 2002 a and the oxide semiconductor film2002 c each have a thickness of greater than or equal to 3 nm and lessthan or equal to 100 nm, preferably greater than or equal to 3 nm andless than or equal to 50 nm. The oxide semiconductor film 2002 b has athickness of greater than or equal to 3 nm and less than or equal to 200nm, preferably greater than or equal to 3 nm and less than or equal to100 nm, further preferably greater than or equal to 3 nm and less thanor equal to 50 nm.

The three semiconductor films (the oxide semiconductor films 2002 a to2002 c) can be either amorphous or crystalline. However, when the oxidesemiconductor film 2002 b where a channel region is formed iscrystalline, the transistor 2000 can have stable electricalcharacteristics; therefore, the oxide semiconductor film 2002 b ispreferably crystalline.

Note that a channel formation region refers to a region of thesemiconductor film of the transistor 2000, which overlaps with a gateelectrode and which is between a source electrode and a drain electrode.Further, a channel region refers to a region through which currentmainly flows in the channel formation region.

For example, when an In—Ga—Zn oxide film formed by a sputtering methodis used as each of the oxide semiconductor films 2002 a and 2002 c, theoxide semiconductor films 2002 a and 2002 c can be deposited with theuse of an In—Ga—Zn oxide target containing In, Ga, and Zn with an atomicratio of 1:3:2. The deposition conditions can be as follows: an argongas (flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm) are usedas the deposition gas; the pressure is 0.4 Pa; the substrate temperatureis 200° C.; and the DC power is 0.5 kW.

When the oxide semiconductor film 2002 b is a CAAC-OS film, the oxidesemiconductor film 2002 b is preferably deposited with the use of apolycrystalline In—Ga—Zn oxide containing In, Ga, and Zn with an atomicratio of 1:1:1. The deposition conditions can be as follows: an argongas (flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm) are usedas the deposition gas; the pressure is 0.4 Pa; the substrate temperatureis 300° C.; and the DC power is 0.5 kW. When the oxide semiconductorfilm 2002 b is a CAAC-OS film, the oxide semiconductor film 2002 b maybe deposited with the use of an In—Ga—Zn oxide target with an atomicratio of 2:1:3. In the CAAC-OS film deposited with the use of thetarget, the proportion of a region where a diffraction pattern of theCAAC-OS film is observed in a predetermined area (also referred to asproportion of CAAC) can be high. As a result, the frequencycharacteristics of a transistor including a channel formation region inthe CAAC-OS film can be high.

Note that the oxide semiconductor films 2002 a to 2002 c can be formedusing a sputtering method.

There are few carrier generation sources in a highly purified oxidesemiconductor (purified oxide semiconductor) obtained by reduction ofimpurities such as moisture and hydrogen serving as electron donors(donors) and reduction of oxygen vacancies; therefore, the highlypurified oxide semiconductor can be an intrinsic (i-type) semiconductoror a substantially i-type semiconductor. For this reason, a transistorhaving a channel formation region in a highly purified oxidesemiconductor film has extremely small off-state current and highreliability. Thus, a transistor in which a channel formation region isformed in the oxide semiconductor film easily has an electricalcharacteristic of a positive threshold voltage (also referred to as anormally-off characteristic).

Specifically, various experiments can prove a small off-state current ofa transistor having a channel formation region in a highly purifiedoxide semiconductor. For example, even when an element has a channelwidth of 1×10⁶ μm and a channel length of 10 μm, off-state current canbe less than or equal to the measurement limit of a semiconductorparameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at voltage(drain voltage) between the source electrode and the drain electrode offrom 1 V to 10 V. In that case, it can be seen that off-state currentstandardized on the channel width of the transistor is lower than orequal to 100 zA/μm. In addition, a capacitor and a transistor areconnected to each other and the off-state current is measured with acircuit in which charge flowing into or from the capacitor is controlledby the transistor. In the measurement, a highly-purified oxidesemiconductor film was used for a channel formation region of thetransistor, and the off-state current of the transistor was measuredfrom a change in the amount of electrical charge of the capacitor perunit hour. As a result, it was found that, in the case where the voltagebetween the source electrode and the drain electrode of the transistoris 3 V, a lower off-state current of several tens of yA/μm is obtained.Accordingly, the off-state current of the transistor in which thepurified oxide semiconductor film is used as a channel formation regionis considerably lower than that of a transistor in which silicon havingcrystallinity is used.

In the case where an oxide semiconductor film is used as thesemiconductor film, at least indium (In) or zinc (Zn) is preferablyincluded as an oxide semiconductor. In addition, as a stabilizer forreducing variations in electrical characteristics among transistorsformed using such an oxide semiconductor, gallium (Ga) is preferablycontained in addition to In and Zn. Tin (Sn) is preferably contained asa stabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer. Zirconium (Zr) ispreferably contained as a stabilizer.

Among oxide semiconductors, unlike silicon carbide, gallium nitride, orgallium oxide, an In—Ga—Zn oxide, an In—Sn—Zn oxide, or the like has anadvantage of high mass productivity because a transistor with favorableelectrical characteristics can be formed by a sputtering method or a wetprocess. Further, unlike silicon carbide, gallium nitride, or galliumoxide, with the use of the In—Ga—Zn oxide, a transistor with favorableelectrical characteristics can be formed over a glass substrate.Further, a larger substrate can be used.

As another stabilizer, one or more lanthanoids selected from lanthanum(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) maybe contained.

As the oxide semiconductor, any of the following oxides can be used, forexample: indium oxide, gallium oxide, tin oxide, zinc oxide, an In—Znoxide, an Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, an Sn—Mg oxide, anIn—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide (also referred to asIGZO), an In—Al—Zn oxide, an In—Sn—Zn oxide, an Sn—Ga—Zn oxide, anAl—Ga—Zn oxide, an Sn—Al—Zn oxide, an In—Hf—Zn oxide, an In—La—Zn oxide,an In—Pr—Zn oxide, an In—Nd—Zn oxide, an In—Ce—Zn oxide, an In—Sm—Znoxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, an In—Tb—Zn oxide, anIn—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide, an In—Tm—Zn oxide,an In—Yb—Zn oxide, an In—Lu—Zn oxide, an In—Sn—Ga—Zn oxide, anIn—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide, anIn—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide.

For example, an In—Ga—Zn oxide refers to an oxide containing In, Ga, andZn, and there is no limitation on the ratio of In to Ga and Zn. Further,the In—Ga—Zn oxide may contain a metal element other than In, Ga, andZn. The In—Ga—Zn oxide has sufficiently high resistance when no electricfield is applied thereto, so that off-state current can be sufficientlyreduced. Moreover, the In—Ga—Zn oxide has high mobility.

For example, high mobility can be obtained relatively easily in the caseof using an In—Sn—Zn oxide. Meanwhile, when an In—Ga—Zn oxide is used,the mobility can be increased by reducing the defect density in a bulk.

In the transistor 2000, a metal in the source and drain electrodes mightextract oxygen from the oxide semiconductor film depending on aconductive material used for the source and drain electrodes. In such acase, a region of the oxide semiconductor film in contact with thesource electrode or the drain electrode becomes an n-type region due tothe formation of an oxygen vacancy. The n-type region serves as a sourceregion or a drain region, resulting in a decrease in the contractresistance between the oxide semiconductor film and the source electrodeor the drain electrode. Accordingly, the formation of the n-type regionincreases the mobility and on-state current of the transistor 2000,achieving the high-speed operation of a semiconductor device using thetransistor 2000.

Note that the extraction of oxygen by a metal in the source electrodeand the drain electrode is probably caused when the source electrode andthe drain electrode are formed by a sputtering method or when heattreatment is performed after the formation of the source electrode andthe drain electrode. The n-type region is more likely to be formed byforming the source electrode and the drain electrode with use of aconductive material that is easily bonded to oxygen. Examples of such aconductive material include Al, Cr, Cu, Ta, Ti, Mo, and W.

In the case where the semiconductor film including the stacked oxidesemiconductor films is used in the transistor 2000, the n-type regionpreferably extends to the oxide semiconductor film 2002 b serving as achannel region in order that the mobility and on-state current of thetransistor 2000 can be further increased and the semiconductor devicecan operate at higher speed.

The insulating film 2001 preferably has a function of supplying part ofoxygen to the oxide semiconductor films 2002 a to 2002 c by heating. Itis preferable that the number of defects in the insulating film 2001 besmall, and typical spin density at g=2.001 due to a dangling bond ofsilicon be lower than or equal to 1×10¹⁸ spins/cm³. The spin density ismeasured by electron spin resonance (ESR) spectroscopy.

The insulating film 2001, which has a function of supplying part ofoxygen to the oxide semiconductor films 2002 a to 2002 c by heating, ispreferably an oxide. Examples of the oxide include aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating film 2001 can be formed by a plasma CVD method, a sputteringmethod, or the like.

Note that in this specification, oxynitride contains more oxygen thannitrogen, and nitride oxide contains more nitrogen than oxygen.

Note that in the transistor 2000 illustrated in FIGS. 16A to 16C andFIGS. 17A to 17C, the conductive film 2006 overlaps with end portions ofthe oxide semiconductor film 2002 b including a channel region that donot overlap with the conductive films 2003 and 2004, i.e., end portionsof the oxide semiconductor film 2002 b that are in a region differentfrom a region where the conductive films 2003 and 2004 are located. Whenthe end portions of the oxide semiconductor film 2002 b are exposed toplasma by etching for forming the end portions, a chlorine radical, afluorine radical, or other radicals generated from an etching gas areeasily bonded to a metal element contained in an oxide semiconductor.For this reason, in the end portions of the oxide semiconductor film,oxygen bonded to the metal element is easily released, so that an oxygenvacancy is easily formed; thus, the end portions of the oxidesemiconductor film easily have n-type conductivity. However, an electricfield applied to the end portions can be controlled by controlling thepotential of the conductive film 2006 because the end portions of theoxide semiconductor film 2002 b that do not overlap with the conductivefilms 2003 and 2004 overlap with the conductive film 2006 in thetransistor 2000 illustrated in FIGS. 16A to 16C and FIGS. 17A to 17C.Consequently, current that flows between the conductive films 2003 and2004 through the end portions of the oxide semiconductor film 2002 b canbe controlled by the potential applied to the conductive film 2006. Sucha structure of the transistor 2000 is referred to as a surroundedchannel (s-channel) structure.

With the s-channel structure, specifically, when a potential at whichthe transistor 2000 is turned off is supplied to the conductive film2006, the amount of off-state current that flows between the conductivefilms 2003 and 2004 through the end portions can be reduced. For thisreason, in the transistor 2000, even when the distance between theconductive films 2003 and 2004 at the end portions of the oxidesemiconductor film 2002 b is reduced as a result of reducing the channellength to obtain high on-state current, the transistor 2000 can have alow off-state current. Consequently, with the short channel length, thetransistor 2000 can have a high on-state current when in an on state anda low off-state current when in an off state.

With the s-channel structure, specifically, when a potential at whichthe transistor 2000 is turned on is supplied to the conductive film2006, the amount of current that flows between the conductive films 2003and 2004 through the end portions of the oxide semiconductor film 2002 bcan be increased. The current contributes to an increase in thefield-effect mobility and an increase in the on-state current of thetransistor 2000. When the end portions of the oxide semiconductor film2002 b overlap with the conductive film 2006, carriers flow in a wideregion of the oxide semiconductor film 2002 b without being limited to aregion in the vicinity of the interface between the oxide semiconductorfilm 2002 b and the insulating film 2005, which results in an increasein the amount of carrier movement in the transistor 2000. As a result,the on-state current of the transistor 2000 is increased, and thefield-effect mobility is increased to greater than or equal to 10 cm²N·sor to greater than or equal to 20 cm²N·s, for example. Note that here,the field-effect mobility is not an approximate value of the mobility asthe physical property of the oxide semiconductor film but is theapparent field-effect mobility in a saturation region of the transistor,which is an index of current drive capability.

<Structure of Oxide Semiconductor Film>

A structure of an oxide semiconductor film will be described below. Notethat in the following description, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. The term “perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 80° and less thanor equal to 100°, and accordingly also includes the case where the angleis greater than or equal to 85° and less than or equal to 95°. Further,the trigonal and rhombohedral crystal systems are included in thehexagonal crystal system in this specification.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of aCAAC-OS film, a polycrystalline oxide semiconductor film, amicrocrystalline oxide semiconductor film, an amorphous oxidesemiconductor film, and the like.

<CAAC-OS Film>

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image(also referred to as a high-resolution TEM image) of a bright-fieldimage and a diffraction pattern of the CAAC-OS film is observed.Consequently, a plurality of crystal parts can be observed. However, inthe high-resolution TEM image, a boundary between crystal parts, thatis, a grain boundary is not clearly observed. Thus, in the CAAC-OS film,a reduction in electron mobility due to the grain boundary is lesslikely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to thesample surface, metal atoms are arranged in a layered manner in thecrystal parts. Each metal atom layer has a form reflecting unevenness ofa surface over which the CAAC-OS film is formed (hereinafter, a surfaceover which the CAAC-OS film is formed is referred to as a formationsurface) or the top surface of the CAAC-OS film, and is arrangedparallel to the formation surface or the top surface of the CAAC-OSfilm.

According to the high-resolution plan-view TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface, metal atoms are arranged in a triangular or hexagonalconfiguration in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is assigned to the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak may also be observed when 2θ is around36°, in addition to the peak at 2θ of around 31°. The peak at 2θ ofaround 36° indicates that a crystal having no c-axis alignment isincluded in part of the CAAC-OS film. It is preferable that in theCAAC-OS film, a peak appear when 2θ is around 31° and that a peak notappear when 2θ is around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Therefore, atransistor including the oxide semiconductor film rarely has negativethreshold voltage (is rarely normally on). The highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasfew carrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electric characteristics andhigh reliability. Electric charge trapped by the carrier traps in theoxide semiconductor film takes a long time to be released and mightbehave like fixed electric charge. Thus, the transistor including theoxide semiconductor film having high impurity concentration and a highdensity of defect states has unstable electric characteristics in somecases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

<Microcrystalline Oxide Semiconductor Film>

Next, a microcrystalline oxide semiconductor film is described.

A microcrystalline oxide semiconductor film has a region in which acrystal part is observed and a region in which a crystal part is notclearly observed in a high-resolution TEM image. In most cases, the sizeof a crystal part included in the microcrystalline oxide semiconductorfilm is greater than or equal to 1 nm and less than or equal to 100 nm,or greater than or equal to 1 nm and less than or equal to 10 nm. Anoxide semiconductor film including a nanocrystal (nc) that is amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as a nanocrystalline oxidesemiconductor (nc-OS) film. In a high-resolution TEM image of the nc-OSfilm, for example, a grain boundary is not clearly observed in somecases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different crystal parts in thenc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak indicating acrystal plane does not appear. Further, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter (e.g., 50 nm or larger)larger than the diameter of a crystal part. Meanwhile, spots are shownin a nanobeam electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter close to or smaller thanthe diameter of a crystal part. Furthermore, in a nanobeam electrondiffraction pattern of the nc-OS film, regions with high luminance in acircular (ring) pattern are shown in some cases. Moreover, in a nanobeamelectron diffraction pattern of the nc-OS film, a plurality of spots isshown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularityas compared with an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film.Therefore, the nc-OS film has a higher density of defect states than theCAAC-OS film.

<Amorphous Oxide Semiconductor Film>

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film is an oxide semiconductor filmhaving disordered atomic arrangement and no crystal part and exemplifiedby an oxide semiconductor film which exists in an amorphous state asquartz.

In a high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is observed whenthe amorphous oxide semiconductor film is subjected to electrondiffraction. Furthermore, a spot is not observed and only a halo patternappears when the amorphous oxide semiconductor film is subjected tonanobeam electron diffraction.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may beseen. Furthermore, in the high-resolution TEM image, there are a regionwhere a crystal part is clearly observed and a region where a crystalpart is not observed. In the a-like OS film, crystallization by a slightamount of electron beam used for TEM observation occurs and growth ofthe crystal part is found sometimes. In contrast, crystallization by aslight amount of electron beam used for TEM observation is hardlyobserved in the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit cell of the InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are layered in the c-axis direction. Accordingly, thespacing between these adjacent layers is substantially equivalent to thelattice spacing (also referred to as d value) on the (009) plane, and is0.29 nm according to crystal structure analysis. Thus, each of thelattice fringes in which the spacing therebetween is from 0.28 nm to0.30 nm corresponds to the a-b plane of the InGaZnO₄ crystal, focusingon the lattice fringes in the high-resolution TEM image.

Note that an oxide semiconductor may be a stacked film including two ormore films of an amorphous oxide semiconductor film, an a-like OS film,a microcrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and the like.

Embodiment 5

In this embodiment, an example of a semiconductor device having astructure different from that in FIG. 15 will be described.

FIG. 18 illustrates an example of a cross-sectional structure of asemiconductor device. In FIG. 18, a region along dashed line A1-A2 showsa structure of the transistors 620 and 630 in the channel lengthdirection, and a region along dashed line A3-A4 shows a structure of thetransistors 620 and 630 in the channel width direction. In oneembodiment of the present invention, the channel length direction of thetransistor 620 is not necessarily aligned with the channel lengthdirection of the transistor 630.

The channel length direction refers to a direction in which a carriermoves between a source (source region or source electrode) and a drain(drain region or drain electrode), and the channel width directionrefers to a direction perpendicular to the channel length direction in aplane parallel to a substrate.

FIG. 18 illustrates the case where the transistor 630 that is an OStransistor is formed over the transistor 620 that is a transistorincluding a material other than an oxide semiconductor in its channelformation region.

Note that such a structure in which the transistor including thematerial other than an oxide semiconductor in its channel formationregion and the OS transistor are stacked can be appropriately used forany of transistors included in the various circuit illustrated in FIGS.1A to 1C, FIGS. 2A and 2B, FIGS. 3A to 3D, FIG. 5, FIGS. 6A to 6C, FIGS.7A to 7C, FIG. 8, FIG. 9, FIG. 10, FIGS. 11A and 11B, and FIGS. 12A to12D.

In this embodiment, as in FIG. 13D, the gate of the transistor 620 isconnected to one of the source and the drain of the transistor 630;however, the structure of the present invention is not limited to this.One of the source and the drain of the transistor 620 may be connectedto the gate of the transistor 630 (see FIG. 14A), one of the source andthe drain of the transistor 620 may be connected to one of the sourceand the drain of the transistor 630 (see FIG. 14B), or the gate of thetransistor 620 may be connected to the gate of the transistor 630 (seeFIG. 14C).

The transistor 620 may include the channel formation region in asemiconductor film or a semiconductor substrate of silicon, germanium,or the like in an amorphous, microcrystalline, polycrystalline, orsingle crystal state. Alternatively, the transistor 620 may include thechannel formation region in an oxide semiconductor film or an oxidesemiconductor substrate. In the case where channel formation regions ofall the transistors are included in an oxide semiconductor film or anoxide semiconductor substrate, the transistor 630 is not necessarilystacked over the transistor 620, and the transistors 630 and 620 may beformed in the same layer.

In the case where the transistor 620 is formed using a thin siliconfilm, any of the following can be used in the thin film: amorphoussilicon formed by a sputtering method or a vapor phase growth methodsuch as a plasma CVD method; polycrystalline silicon obtained bycrystallization of amorphous silicon by treatment such as laserirradiation; single crystal silicon obtained by separation of a surfaceportion of a single crystal silicon wafer by implantation of hydrogenions or the like into the silicon wafer; and the like.

A substrate 1000 over which the transistor 620 is formed can be, forexample, a silicon substrate, a germanium substrate, or a silicongermanium substrate. In FIG. 18, a single crystal silicon substrate isused as the substrate 1000.

The transistor 620 is electrically isolated by element isolation. As theelement isolation method, a trench isolation method (a shallow trenchisolation (STI) method) or the like is used. FIG. 18 illustrates anexample where the trench isolation method is used to electricallyisolate the transistor 620. Specifically, in FIG. 18, the transistor 620is electrically isolated by element isolation using an element isolationregion 1001 formed in such a manner that an insulator including siliconoxide or the like is buried in a trench formed in the substrate 1000 byetching or the like and then the insulator is removed partly by etchingor the like.

In a projection of the substrate 1000 that exists in a region other thanthe trench, an impurity region 1002 and an impurity region 1003 of thetransistor 620 and a channel formation region 1004 placed between theimpurity regions 1002 and 1003 are provided. Furthermore, the transistor620 includes an insulating film 1005 covering the channel formationregion 1004 and a gate electrode 1006 that overlaps with the channelformation region 1004 with the insulating film 1005 providedtherebetween.

In the transistor 620, a side portion and an upper portion of theprojection in the channel formation region 1004 overlap with the gateelectrode 1006 with the insulating film 1005 positioned therebetween, sothat carriers flow in a wide area including the side portion and theupper portion of the channel formation region 1004. Therefore, thenumber of transferred carriers in the transistor 620 can be increasedwhile an area over the substrate occupied by the transistor 620 isreduced. As a result, the on-state current and field-effect mobility ofthe transistor 620 are increased. Suppose the length in the channelwidth direction (channel width) of the projection in the channelformation region 1004 is W, and the thickness of the projection in thechannel formation region 1004 is T. When the aspect ratio of thethickness T to the channel width W is high, a region where carriers flowbecomes larger. Thus, the on-state current of the transistor 620 can befurther increased and the field-effect mobility of the transistor 620can be further increased.

Note that when the transistor 620 is formed using a bulk semiconductorsubstrate, the aspect ratio is desirably 0.5 or more, further desirably1 or more.

An insulating film 1011 is provided over the transistor 620. Openingportions are formed in the insulating film 1011. Conductive films 1012and 1013 that are electrically connected to the impurity regions 1002and 1003, respectively, and a conductive film 1014 that is electricallyconnected to the gate electrode 1006 are formed in the opening portions.

The conductive film 1012 is electrically connected to a conductive film1016 formed over the insulating film 1011. The conductive film 1013 iselectrically connected to a conductive film 1017 formed over theinsulating film 1011. The conductive film 1014 is electrically connectedto a conductive film 1018 formed over the insulating film 1011.

An insulating film 1020 is provided over the conductive films 1016 to1018. An insulating film 1021 having a blocking effect of preventingdiffusion of oxygen, hydrogen, and water is provided over the insulatingfilm 1020. As the insulating film 1021 has higher density and becomesdenser or has a fewer dangling bonds and becomes more chemically stable,the insulating film 1021 has a higher blocking effect. The insulatingfilm 1021 that has the effect of blocking diffusion of oxygen, hydrogen,and water can be formed using, for example, aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, or hafnium oxynitride. The insulating film1021 having an effect of blocking diffusion of hydrogen and water can beformed using, for example, silicon nitride or silicon nitride oxide.

An insulating film 1022 is provided over the insulating film 1021, andthe transistor 630 is provided over the insulating film 1022.

The transistor 630 includes, over the insulating film 1022, asemiconductor film 1030 including an oxide semiconductor, conductivefilms 1032 and 1033 functioning as source and drain electrodes andelectrically connected to the semiconductor film 1030, a gate insulatingfilm 1031 covering the semiconductor film 1030, and a gate electrode1034 overlapping with the semiconductor film 1030 with the gateinsulating film 1031 positioned therebetween. Note that an opening isformed in the insulating films 1020 to 1022. The conductive film 1033 isconnected to the conductive film 1018 in the opening.

Note that in FIG. 18, the transistor 630 includes at least the gateelectrode 1034 on one side of the semiconductor film 1030, and mayfurther include a gate electrode overlapping with the semiconductor film1030 with the insulating film 1022 positioned therebetween.

In the case where each of the transistor 630 has a pair of gateelectrodes, one of the gate electrodes may be supplied with a signal forcontrolling the on/off state, and the other of the gate electrodes maybe supplied with a potential from another element. In this case,potentials with the same level may be supplied to the pair of gateelectrodes, or a fixed potential such as the ground potential may besupplied only to the other of the gate electrodes. By controlling thelevel of a potential supplied to the other of the gate electrodes, thethreshold voltage of the transistor can be controlled.

In FIG. 18, the transistor 630 has a single-gate structure where onechannel formation region corresponding to one gate electrode 1034 isprovided. However, the transistor 630 may have a multi-gate structurewhere a plurality of channel formation regions are formed in one activelayer by providing a plurality of gate electrodes electrically connectedto each other.

FIG. 18 illustrates an example in which the semiconductor film 1030included in the transistor 630 includes oxide semiconductor films 1030 ato 1030 c that are stacked in this order over the insulating film 1022.Note that in one embodiment of the present invention, the semiconductorfilm 1030 of the transistor 630 may be formed using a single-layer metaloxide film.

Note that this embodiment can be implemented in appropriate combinationwith other embodiments.

Embodiment 6

The variety of films disclosed in the other embodiments, such as theconductive films, the semiconductor films, and the insulating films canbe formed by a sputtering method or a plasma CVD method; however, suchfilms may be formed by another method, e.g., a thermal CVD (chemicalvapor deposition) method. A metal organic chemical vapor deposition(MOCVD) method or an atomic layer deposition (ALD) method may beemployed as an example of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is caused in the vicinity of thesubstrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, an inert gas (e.g., argon ornitrogen) or the like is introduced when or after a first source gas isintroduced so that the source gases are not mixed, and then a secondsource gas is introduced. Note that in the case where the inert gas isintroduced at the same time as the first source gas, the inert gasserves as a carrier gas, and the inert gas may also be introduced at thesame time as the second source gas. Alternatively, the first source gasmay be exhausted by vacuum evacuation instead of the introduction of theinert gas, and then the second source gas may be introduced. The firstsource gas is adsorbed on the surface of the substrate to form a firstlayer; then the second source gas is introduced to react with the firstlayer; as a result, a second layer is stacked over the first layer, sothat a thin film is formed. The sequence of the gas introduction isrepeated plural times until a desired thickness is obtained, whereby athin film with excellent step coverage can be formed. The thickness ofthe thin film can be adjusted by the number of repetition times of thesequence of the gas introduction; therefore, an ALD method makes itpossible to accurately adjust a thickness and thus is suitable formanufacturing a minute FET.

The variety of films such as the conductive film, the semiconductorfilm, and the insulating film which have been disclosed in theembodiments can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O film isformed, trimethylindium, trimethylgallium, and dimethylzinc can be used.Note that the chemical formula of trimethylindium is In(CH₃)₃. Thechemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formulaof dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed with adeposition apparatus employing ALD, two kinds of gases, i.e., ozone (O₃)as an oxidizer and a source material gas which is obtained by vaporizingliquid containing a solvent and a hafnium precursor compound (hafniumalkoxide or hafnium amide such as tetrakis(dimethylamide)hafnium(TDMAH)) are used. Note that the chemical formula oftetrakis(dimethylamide)hafnium is Hf[(CH₃)₂]₄. Examples of anothermaterial liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, e.g., H₂O as anoxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed with adeposition apparatus employing ALD, hexachlorodisilane is adsorbed on asurface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times toform an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at atime to form a Ga—O layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a Zn—O layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas which is obtainedby bubbling with an inert gas such as Ar may be used instead of an O₃gas, it is preferable to use an O₃ gas, which does not contain H.Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of aGa(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gasmay be used.

Note that this embodiment can be implemented in appropriate combinationwith other embodiments.

Embodiment 7

In Embodiment 7, application examples of the semiconductor deviceaccording to one embodiment of the present invention will be described.

The semiconductor device according to one embodiment of the presentinvention can acquire predetermined physical quantities or predeterminedchemical quantities as shown in the above embodiments. Thus, biologicaldata can be obtained anywhere and anytime when persons, animals, or thelike carry the semiconductor device.

As a method for carrying the semiconductor device, in the case of aperson for example, attaching a semiconductor device to the surface ofthe body or implanting it in the body of the person can be considered.However, the method can be appropriately selected in consideration ofphysical quantities or chemical quantities to be acquired. FIGS. 19A to19E illustrate specific application examples of the semiconductor deviceof the present invention.

FIG. 19A is a bangle-type electronic device 5001, and a semiconductordevice 5003 is provided in a housing 5002. The electronic device 5001 isworn by a user so that the semiconductor device 5003 is in contact withthe wrist or the arm of the user, whereby biological data can beacquired from the wrist or the arm of the user. Note that the electronicdevice 5001 can also be worn on the waist or the leg. A belt or the likecan be used instead of the housing 5002. The biological data acquired bythe semiconductor device 5003 can be read out by a reader/writer, or thelike.

The semiconductor device can also be implanted in the body of the user.FIG. 19B illustrates an application example when the semiconductordevice 5004 is implanted in the wrist. In this case, the semiconductordevice 5004 can be worn without using a housing or a belt, so that laborfor attaching/detaching thereof can be eliminated. Note that thesemiconductor device 5004 can be implanted into any portion of the humanbody such as in the mouth or the earlobe without limitation to the wristof the user (FIG. 19C).

As shown in FIG. 19D, the semiconductor device 5004 can be attached toor implanted into an animal. The biological data of the animal acquiredby the semiconductor device 5004 is read out regularly, so that thehealth condition of the animal can be monitored and managed. In thiscase, it is possible to manage a plurality of animals simultaneously bymaking the semiconductor device 5004 store identification numbers inadvance.

As shown in FIG. 19E, the semiconductor device 5004 can be attached toor implanted into a plant. The biological data of the plant acquired bythe semiconductor device 5004 is read out regularly, so that data onflowering time, shipping time, and the like can be expected. When thesemiconductor device 5004 includes an element for detecting light, dataon sunshine duration can be obtained. When the semiconductor device 5004includes a solar cell, light from the outside is converted into electricpower and the electric power is supplied to the semiconductor device5004, whereby the semiconductor device 5004 can operate.

As described above, the semiconductor device according to one embodimentof the present invention can be attached to or implanted into the livingthings such as persons, animals and plants, so that biological data ofan individual living things can be easily acquired.

Application examples of this invention are not limited to thosedescribed above. The semiconductor device of this invention can beapplied to various electronic devices such as display devices, personalcomputers, image reproducing devices provided with recording media(typically, devices which reproduce the content of recording media suchas digital versatile discs (DVDs) and have displays for displayingreproduced images), cellular phones, portable game machines, personaldigital assistants, e-book readers, cameras such as video cameras anddigital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), vending machines, and medicaldevices.

Note that this embodiment can be implemented in appropriate combinationwith other embodiments.

This application is based on Japanese Patent Application serial no.2014-105748 filed with Japan Patent Office on May 22, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firstcircuit configured to acquire data from an outside; a second circuitconfigured to convert the data into a digital signal; and a thirdcircuit comprising a first wiring, a second wiring, a third wiring, afourth circuit, and an arithmetic circuit, wherein the fourth circuitoverlaps with at least a part of the arithmetic circuit, wherein thefourth circuit includes a first memory circuit and a second memorycircuit, wherein a structure of the first memory circuit and a structureof the second memory circuit are different from each other, wherein thefirst wiring is electrically connected to the first memory circuit,wherein the second wiring is electrically connected to the second memorycircuit, wherein the third wiring is electrically connected to the firstmemory circuit and the second memory circuit, wherein the second memorycircuit is electrically connected to the arithmetic circuit, wherein thefirst memory circuit includes a first transistor including an oxidesemiconductor in a channel formation region, wherein the second memorycircuit includes a second transistor including an oxide semiconductor ina channel formation region, wherein the first memory circuit isconfigured to store the data, wherein the second memory circuit isconfigured to store reference data, and wherein the arithmetic circuitis configured to compare the data and the reference data.
 2. Thesemiconductor device according to claim 1, wherein the second memorycircuit includes a capacitor and an inverter, wherein one of a sourceand a drain of the first transistor is electrically connected to thecapacitor and an input terminal of the inverter, and wherein an outputterminal of the inverter is electrically connected to the arithmeticcircuit.
 3. The semiconductor device according to claim 1, furthercomprising a third transistor, wherein one of a source and a drain ofthe third transistor is electrically connected to the fourth circuit,wherein the other of the source and the drain of the third transistor iselectrically connected to the arithmetic circuit, and wherein the thirdtransistor includes an oxide semiconductor in a channel formationregion.
 4. The semiconductor device according to claim 3, wherein theoxide semiconductor of the third transistor is provided in a same layeras the oxide semiconductor of the first transistor.
 5. The semiconductordevice according to claim 1, further comprising a fifth circuitconfigured to transmit and receive a wireless signal.
 6. A semiconductordevice comprising: a first circuit configured to acquire biologicaldata; a second circuit configured to convert the biological data into adigital signal; and a third circuit comprising a first wiring, a secondwiring, a third wiring, a first memory circuit, a second memory circuitand an arithmetic circuit, wherein a structure of the first memorycircuit and a structure of the second memory circuit are different fromeach other, wherein one of the first memory circuit and the secondmemory circuit overlaps with at least a part of the arithmetic circuit,wherein the first wiring is electrically connected to the first memorycircuit, wherein the second wiring is electrically connected to thesecond memory circuit, wherein the third wiring is electricallyconnected to the first memory circuit and the second memory circuit,wherein the second memory circuit is electrically connected to thearithmetic circuit, wherein the first memory circuit includes a firsttransistor including an oxide semiconductor in a channel formationregion, wherein the second memory circuit includes a second transistorincluding an oxide semiconductor in a channel formation region, whereinthe first memory circuit is configured to store the biological data,wherein the second memory circuit is configured to store reference data,and wherein the arithmetic circuit is configured to compare thebiological data and the reference data.
 7. The semiconductor deviceaccording to claim 6, wherein the first memory circuit includes a firstcapacitor, wherein one of a source and a drain of the first transistoris electrically connected to the first capacitor, wherein the secondmemory circuit includes a second capacitor and an inverter, wherein oneof a source and a drain of the second transistor is electricallyconnected to the second capacitor and an input terminal of the inverter,and wherein an output terminal of the inverter is connected to thearithmetic circuit.
 8. The semiconductor device according to claim 6,further comprising a third transistor, wherein one of a source and adrain of the third transistor is electrically connected to the firstmemory circuit, wherein the other of the source and the drain of thethird transistor is electrically connected to the arithmetic circuit,and wherein the third transistor includes an oxide semiconductor in achannel formation region.
 9. The semiconductor device according to claim8, wherein the oxide semiconductor of the third transistor is providedin a same layer as the oxide semiconductor of the first transistor. 10.The semiconductor device according to claim 6, wherein the oxidesemiconductor of the second transistor is provided in a same layer asthe oxide semiconductor of the first transistor.
 11. The semiconductordevice according to claim 6, further comprising a fourth circuitconfigured to transmit and receive a wireless signal.
 12. Asemiconductor device comprising: a first circuit configured to acquirebiological data; a second circuit configured to convert the biologicaldata into a digital signal; and a third circuit comprising a firstwiring, a second wiring, a third wiring, a first memory circuit, asecond memory circuit and an arithmetic circuit, wherein a structure ofthe first memory circuit and a structure of the second memory circuitare different from each other, wherein the arithmetic circuit comprisesa fourth circuit and a fifth circuit, wherein one of the first memorycircuit and the second memory circuit overlaps with at least a part ofthe arithmetic circuit, wherein the first wiring is electricallyconnected to the first memory circuit, wherein the second wiring iselectrically connected to the second memory circuit, wherein the thirdwiring is electrically connected to the first memory circuit and thesecond memory circuit, wherein the second memory circuit is electricallyconnected to the fourth circuit, wherein the fourth circuit iselectrically connected to the fifth circuit, wherein the first memorycircuit includes a first transistor including an oxide semiconductor ina channel formation region, wherein the second memory circuit includes asecond transistor including an oxide semiconductor in a channelformation region, wherein the first memory circuit is configured tostore the biological data, wherein the second memory circuit isconfigured to store reference data, wherein the fourth circuit isconfigured to compare the biological data and the reference data, andwherein the fifth circuit is configured to output an interrupt signal.13. The semiconductor device according to claim 12, wherein the firstmemory circuit includes a first capacitor, wherein one of a source and adrain of the first transistor is electrically connected to the firstcapacitor, wherein the second memory circuit includes a second capacitorand an inverter, wherein one of a source and a drain of the secondtransistor is electrically connected to the second capacitor and aninput terminal of the inverter, and wherein an output terminal of theinverter is connected to the arithmetic circuit.
 14. The semiconductordevice according to claim 12, further comprising a third transistor,wherein one of a source and a drain of the third transistor iselectrically connected to the first memory circuit, wherein the other ofthe source and the drain of the third transistor is electricallyconnected to the fourth circuit, and wherein the third transistorincludes an oxide semiconductor in a channel formation region.
 15. Thesemiconductor device according to claim 14, wherein the oxidesemiconductor of the third transistor is provided in a same layer as theoxide semiconductor of the first transistor.
 16. The semiconductordevice according to claim 12, wherein the oxide semiconductor of thesecond transistor is provided in a same layer as the oxide semiconductorof the first transistor.
 17. The semiconductor device according to claim12, further comprising a sixth circuit configured to transmit andreceive a wireless signal.